William J. Beaty 3/1995
WARNING: This file is currently being written, edited, corrected, etc. It does still contain some mistakes of its own. I placed it online as a sort of 'trial by fire' in order to hear readers' responses so I could target weak or unclear sections for improvement. (And, as my site points out, NOBODY is perfect so we should always practice critical thinking. Take all information with a grain of salt, including everything here!) Please feel free to send public comments to me with the COMMENT BOOK. If you prefer that nobody else sees your comments, send private comments to me via this form.
"Lest you think that I am quibbling over minor points of language, I note that in my experience many of the misconceptions people harbor have their origins in imprecise language... Precise language is needed in science, not to please pedants but to avoid absorbing nonsense that will take years, if ever, to purge from our minds." - Dr. Craig F. Bohren, Physicist
"Errors, like straws, upon the surface flow; He who would search for pearls must dive below." - John Dryden
That's the way all the books were: They said things that were useless, mixed-up, ambiguous, confusing, and partially incorrect. How anybody can learn science from these books, I don't know, because it's not science. - RP Feynman, in Judging Books By Their Covers
Want books? Try searching amazon.com:
CORRECT: There is no single list called "The Scientific Method." It is a myth.
See the links and references below.
The rules of a science-fair typically require that students follow THE
SCIENTIFIC METHOD, or in other words, hypothesis-experiment-conclusion.
The students must propose a hypothesis and test it by experiment. This
supposedly is the "Scientific Method" used by all scientists.
Supposedly, if you don't follow the rigidly defined "Scientific Method"
listed in K-6 textbooks, then you're not doing science. (Some science
fairs even ban astronomy and paleontology projects. After all, where's
the "experiment" in these?)
Unfortunately this is wrong, and there is no single "Scientific Method"
as such. Scientists don't follow a rigid procedure-list called "The
Scientific Method" in their daily work. The procedure-list is a myth
spread by K-6 texts. It is an extremely widespread myth, and even some
scientists have been taken in by it, but this doesn't make it any more
real. "The Scientific Method" is part of school and school books, and is
not how science in general is done. Real scientists use a large variety
of methods (perhaps call them methods of science rather than "The
Scientific Method.") Hypothesis / experiment / conclusion is one of
these, and it's very important in experimental science such as physics
and chemistry, but it's certainly not the only method. It would be a
mistake to elevate it above all others. We shouldn't force children to
memorize any such procedure list. And we shouldn't use it to exclude
certain types of projects from science fairs! If "The Scientific Method"
listed in a grade school textbook proves that Astronomy is not a science,
then it's the textbook which is wrong, not Astronomy.
"Ask a scientist what he conceives the scientific method to be and he adopts an expression that is at once solemn and shifty-eyed: solemn, because he feels he ought to declare an opinion; shifty-eyed because he is wondering how to conceal the fact that he has no opinion to declare." - Sir Peter MedawarThere are many parts of science that cannot easily be forced into the mold of "hypothesis-experiment-conclusion." Astronomy is not an experimental science, and Paleontologists don't perform Paleontology experiments... so is it not proper Science if you study stars or classify extinct creatures?
Or, if a scientist has a good idea for designing a brand new kind of
measurement instrument (e.g. Newton and the reflecting telescope)
...that's certainly "doing science." Humphrey Davy says "Nothing tends so
much to the advancement of knowledge as the application of a new
instrument." But where is 'The Hypothesis?' Where is 'The Experiment?' The
Atomic Force Microscope (STM/AFM) revolutionized science. Yet if a mere
science student had actually invented the very first reflector telescope
or the very first AFM, wouldn't such a device be rejected from many school
science fairs? After all, it's not an experiment, and the list named "The
Scientific Method" says nothing about exploratory observation. Some
science teachers would reject the discovery of the Tunneling Microscope as
science; calling it 'mere engineering.' Yet like the Newtonian reflector,
the tunneling microscope is a revolution that opened up an entire new
branch of science. Since it's instrument-inventing, not
hypothesis-testing, must we exclude it as science? Were the creators of
the STM not doing science when they came up with that device? In
defining Science, the Nobel Prize committee disagrees with the science
teachers and science fair judges. The researchers who created the STM won
Nobel Prize in physics. I'd say that if someone wins a Nobel Prize in
the sciences, it's a good bet that their work qualifies as "science."
Forcing kids to follow a caricature of scientific research distorts
science, misleads generations of students, and it really isn't necessary
in the first place.
Another example: great discoveries often come about when scientists notice anomalies. They see something inexplicable during past research, and that triggers some new research. Or sometimes they notice something weird out in Nature; something not covered by modern theory. Isaac Asimov said it well:
"The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny...' "This suggests that lots of important science comes NOT from proposing hypotheses or even from performing experiments, but instead comes from unguided observation and curiosity-driven exploration: from sniffing about while learning to see what nobody else can see. Scientific discovery comes from something resembling "informed messing around," or unguided play. Yet the "Scientific Method" listed in textbooks says nothing about this. Instead their lists start out with "Form an Hypothesis." As a result, educators treat science as UNplayful, deadly serious business. "Messing around" is sometimes dealt with harshly. See: The Onion, science teacher satire.
"Let me state the Method Position as follows: 'There is something called the scientific method, and someone who understands this method will be able to understand all of science, regardless of the specific subject matter that person has been taught. Thus the goal of science education should be to teach that method.' It's hard for me to understand how anyone could hold a position that is so clearly untenable."
"Why should there be the method of science? There is not just one way to build a house, or even to grow tomatoes. We should not expect something as motley as the growth of knowledge to be strapped to one methodology." -Ian Hacking
CORRECT: The ocean is blue because water is a blue substance.
Many people are sure that bodies of water are blue because the water reflects the sky. But wouldn't this only make the shiny surface-reflections look blue? And doesn't water sometimes remain blue on cloudy days? Exactly. There's no mystery here; water looks blue because water *is* blue. Pure water is a blue chemical. It's not just the sky that creates the colors we see.
But what if you pour yourself a drink; in that case the water is clear, right? Well ...it's not blue as far as your eyes can tell. But what if the water in your cup actually was very very slightly blue. You'd never see it. You'd only notice the blue color if your cup was many feet wide.
In fact, that's exactly how it works: pure water is nearly clear, but it's
very very slightly blue. A small amount of water is too thin a layer, so
a small amount looks clear rather than blue. But look through thirty feet
of water, especially with a white sandy bottom, and you'll see a strong
color. Gaze into a hundred feet of deep pure mountain lake water against
a white rocky bottom on a sunny day. You'll see exactly what color the
water actually has. Yet if you scoop a canteen full of that lake water,
it will seem totally clear.
CORRECT: The sky is blue because air is blue.
This one isn't purely a textbook error. Still, it involves misconceptions
on the part of authors.
Why is "the sky" colored blue? Usually the books start going on about
of light, Tyndall effect, and Rayleigh scattering. It's a bit much for
young children. First the books try to teach some correct but complicated
physics. Then they use it to explain blue sky and sunsets. But what
happens when kids don't understand the physics? Doesn't this make the
explanation useless? And do the kids just give up?
It's all wrong: you don't need complicated physics to understand this. The sky is blue for a very simple reason:
The Earth's atmosphere is not a perfectly transparent material. Instead it is blue!Take away the air, and we'd see the same black daytime seen by the lunar astronauts. No air, no blue. There is no "sky" up there; no solid surface. All we're seeing is sunlit air.
A cloud of air looks blue for much the same reason that a cloud of
powder looks white.
Powder isn't invisible. Neither is air. Throw some dust upwards
on a sunny
day and you'll see a visible white cloud. But what happens if you could
throw some AIR? You might think that a cloud of air would be invisible.
You'd be wrong. Air isn't invisible, instead its molecules scatter light
in the same way that any small particles do. Deposit a huge cloud of air
the surface of the airless moon, and you'd see its bright blue color
blackness of hard vacuum. Air is a powdery-blue
substance. (But then... shouldn't air be a white substance. Yes! And
that's where the complicated physics comes in: explaining why air looks
blue rather than white.)
The color of air can be confusing because air seems transparent. Capture
a jar full of air, but you see no color. It's true that small amounts of
air are almost perfectly transparent. But so are small amounts of water.
Go to an opaque muddy river or pond and use a cup to dip out some water.
The water looks fairly clear, no? Yet the deep river is opaque brown.
Whenever you try to look through ten cups of water, or a hundred cups, the
water seems to turn into opaque brown paint. Yet a single cup of river
water almost looks clean.
Air behaves similarly. A mile of air looks clear, but ten miles of
air looks misty blue, and a thousand miles of air looks opaque white.
The air is acting like the dirty river water where a thin layer looks
colorless but a thick layer does not. Air acts like a fogbank where
distant objects are invisible, yet you can see your own hand just
"The sky" is blue because air is a powdery blue material; a collection of
tiny specks, and when the sun
shines upon it, we can see this blue color. Each molecule of air behaves
like mote of dust. Stare upwards on a sunny day, and
you're looking into a thick cloud of brightly-lit air. (Note: there
really is no "sky" up there at all. The sky is an illusory surface.
You're not really looking at a blue surface. There is no "sky"
which is colored blue, instead you're just seeing
the Earth's layer of blue air against the blackness of outer space. )
OK, suppose you could go far out into space away from the Earth, then
build yourself a thin hollow glass bubble a thousand miles wide. Viewed
from the Earth, your empty glass bubble would be almost invisible. OK,
now fill your bubble with air. It won't be invisible any more. It will
look like a giant droplet of bright blue paint. It probably even looks
whitish in the middle, since very thick layers of air seem as white as
milk. What if you let your giant glass bubble crash into the moon? The
air inside would pour out over the moon's surface and form a thick
temporary layer of atmosphere. The moon wouldn't look white anymore. It
would have the same blue borders that Earth has.
Photos of sunlit air, observed against black space:
Ah, if you start asking why air acts like this, *now* you finally need the advanced physics explanations. Many physics books will explain Rayleigh scattering; explain why an air molecule looks like a bluish dust mote, but looks reddish when lit from behind.
CORRECT: Clouds actually remain aloft because they are warm inside.
Clouds are heavy. Evaporated water (the H2O gas) is not heavy, it
less dense than air, so
moist air rises. But when the water-gas condenses to form clouds, it
contracts by about 1000 times and turns into very dense liquid water.
(Imagine that the helium in a balloon condensed into a liquid. Would a
tiny liquid-filled balloon still be buoyant? Nope.)
Even a small
many tons of liquid water. How can clouds remain aloft?
Many sources claim that clouds remain aloft because the water droplets are
so small and widely separated that gravity has less effect on them. This is
wrong. It doesn't matter if you break up a body of water into tiny
droplets; its weight remains the same. You can't fool gravity. If a
cloud contains tons of water, it will be pulled down to the Earth's
surface with the same force whether the water forms a cloud or whether it
forms raindrops. The answer lies elsewhere.
Some sources claim that clouds remain aloft because of updrafts: because
the air had been rising, and the rising air blows the cloud droplets
upwards. Wrong again:
should be quickly halted as soon as the low-density water vapor turns
into a dense liquid. The excess weight will slow the updraft,
stop it, then reverse it. To keep clouds aloft, we'd need some sort of
weirdly constant updraft, or one where thermal energy is being created,
not an updraft that's easily reversed by a falling cloud.
Still other sources claim that clouds stay up there because the droplets
are very tiny, so they settle through the air very slowly. This is true,
but it still doesn't explain how weighty water can remain aloft. Stop and
a bit... if we have hundreds of tons of water, will its weight disappear
simply because it has been divided into tiny droplets? No, instead the
heavy droplets drag the surrounding air downwards as they fall. Air which
water droplets is denser than normal air. Its weight is increased by
almost exactly the weight of the suspended water droplets, which works out
to around 1/10 percent of the weight of the air in a particular volume.)
Dense air falls fast! In other words, the tiny droplets will still race
downwards because they form heavy white cloud-stuff, and both the droplets
the air between them will be dragged downwards by gravity. Anyone playing
with humidifier fog
knows this: dense white pours downwards like a liquid. Yet even some
professional meteorologists are saying these
things about droplets. They should know better.
So why *DO* clouds stay up there? Why don't they pour downwards to form a
ground-hugging fog? The answer is simple: the weight of the cloud's
droplets is countered by the buoyancy of heated air between the
droplets. Clouds are like hot air balloons!
Whenever liquid water condenses from H2O gas, it releases thermal energy.
When moist air turns into droplet-filled air, the droplets are hot, and
they warm the air too. The heated air expands and becomes less dense.
Is this enough to stop the falling droplets? Yes, it's more than enough,
and the warm foggy air flows upwards.
Clouds stay up there because they're significantly less dense on average
than the surrounding air. In fact, if the water droplets should meld
together, then fall out
cloud as rain, then the remaining hot air is no longer weighed down by
tons and tons of water, and it rises even more quickly. This low-density,
moving warm air is the
"engine" which drives the violent updrafts in thunderstorms and
hurricanes. Hot air with its water removed no longer floats serenely
along as clouds, instead it can form upward jets with hurricane velocity.
Try making this "Touch
The Clouds" device and you'll discover that droplet-filled air can be
dense indeed. You can easily pour it from a pitcher and fill some cups.
also know that hot air is less dense that cool air of the same pressure,
so hot must rise through cooler air. Mix the two ideas together: dense
air which is
full of water droplets becomes less dense when heated, and at a certain
higher temperature it should be buoyed upwards by the atmosphere even
it's still full of suspended mass-bearing water droplets. If we could
humidifier-mist warm enough, it would rise and form indoor
More thinking: helium gas rises in air, but liquid helium does not.
Liquid helium is heavy, like liquid water (though not quite as heavy as an
equal quantity of water.)
So, what happens when helium gas
condenses into liquid? It shrinks greatly, becoming more dense than the
surrounding air, then it dribbles downwards like any liquid. It falls
downwards if it's a large blob of liquid, and it falls downward even if it
takes the form of tiny droplets. If the helium in a balloon was changed
into liquid, the balloon would fall. The same is true of water. Water
vapor (h2o gas,) like helium, is lighter than air, and it will rise.
However, if that vapor should condense into droplets, it greatly contracts
in size and greatly increases in density. A cloud of water droplets is
heavy, and on average it should fall downwards. Even if the
that they individually settle slowly, the droplets together have
significant weight, so the droplets should drag the air downwards as they
go. The dense, droplet-filled air may fall quite quickly, even though the
individual droplets remain "stuck in the air" because of forces of
Whenever vapor condenses to form droplets, it releases "heat of
condensation" which causes the remaining air to expand. The warm air can
expand even MORE than the volume left empty by the condensing vapor,
causing the average density to fall and causing clouds to rise upwards
rather than just float. When clouds first form, they usually pour
downwards. They're a bit too warm, so they try to rise to a higher level.
CORRECTED: A single lemon battery cannot light a flashlight bulb
Some gradeschool science books contain "experiments" which do not
prism experiment below is one of them. Another is the "lemon battery"
or "potato battery" used to run a flashlight bulb. If you stick some
copper and zinc
into a single lemon, this "battery" does create a small voltage. Touch
lemon-cell to the wires of a loudspeaker or headphones and you'll hear a
clicking sound. Connect it to an old-style panel meter (a voltmeter or
milliamp-meter; the kind with the
moving needle,) and your lemon can make the meter needle move. Three or
four lemon-cells connected in series can run an LCD digital clock or
light up a red Light Emitting Diode LED. (If you try the digital clock or
LED, remember that polarity is important, and if it doesn't work, try
reversing the connections.)
HOWEVER... the lemon's electrical output is far too feeble to
light up a
standard flashlight bulb. Same with motors, buzzers, etc. The lemon
battery is too weak. The experiment described in the books doesn't
How can I be certain? All those books say one thing, and I'm just
one person who says differently. Doesn't the majority rule? No,
because science is based on reality staying the same, and Nature ignores
what humans vote upon. It doesn't matter how many books say that
lemon batteries can light a flashlight bulb. Nature can't be
Let's look at a real world example: I stick a fairly wide copper
strip and a similar zinc strip into
a lemon. (This works much better than copper pennies or zinc nails.)
Clean the strips with sandpaper beforehand.
First use the strips to tear up the inside of the lemon, then insert the
metal strips very close together to give best results. The area of each
"battery plate" is around 1 inch square. Measured voltage: 0.91V.
Measured short-circuit current: two milliamps (0.002 Amps) immediately
decreasing to a constant half a milliamp (0.0005 amps.) What does this
mean? Well, a typical flashlight bulb draws an ENTIRE QUARTER OF AN
AMPERE when lit. Not a half-milliamp, but 250 milliamps or 0.250 Amps.
To light up a normal flashlight bulb, you'd need
500 lemons wired in parallel! 0.2500amps / 0.0005amps = 500 lemons.
However, there are specialized light bulbs which draw very tiny currents.
Maybe the experiments in the books weren't talking about a standard
flashlight bulb? (Most of them never say. But I'll give them the benefit
of the doubt, although perhaps I shouldn't.)
From Radio Shack we can get a #272-1139 incandescent bulb which only draws
around fifteen milliamps (0.015 amps) at 0.7 volts when lit very dimly in
a darkened room. This is the most sensitive incandescent bulb I've ever
encountered. To light this bulb we only need 0.0150A/0.0005A = 30
lemons wired in parallel. THIRTY LEMONS. And the bulb is so dim that you
can't see the glow unless the room is dark. But wasn't the lemon's
electric current higher
at the start? 0.002 amps, not 0.0005 amps? Yes, so with only TEN LEMONS
wired in parallel, maybe we could cause the special hyper-sensitive light
bulb to blink on for a second or two before going dark.
This still translates into "the experiment doesn't work." One
single lemon cannot
light up any sort of incandescent bulb. At best we can use several lemons
to light an LED.
If a science book contains the lemon battery
bulb-lightning experiment, it means that the author never performed the
experiment to see if it works. LOTS of books and websites say that a
single lemon can light a flashlight bulb. Every single one of these is
wrong. The mistake is like a kind of infection. If you aren't careful,
science website can catch a disease!
Can't we build a larger lemon-juice battery in a jar which will light a
Yes, but your battery needs to be fairly large; much larger than a couple
of metal parts stuck into a lemon. At the very least you'll need a jar
for the juice, plus some sheets of copper and zinc several inches wide.
If you don't have that special Radio Shack bulb, then you'll need more
than one lemon-juice jar hooked in series to make the 1.5 volts needed by
a standard flashlight bulb. (I'll try building one of these and report
back about how large the copper and zinc plates must be.)
If you really want to light up a small lightbulb, why not build an ELECTRIC GENERATOR
How to cheat!There is a secret way to make a lemon-cell light up an incandescent bulb. You have to cheat. Buy yourself a "super capacitor" or "memory backup capacitor" via mail-order surplus. They cost a few dollars. You want a value between 0.1 farad and 0.5 farads. Try one of these suppliers:
It's not an ideal experiment, and it's hard to explain how capacitors
work. But it's easier than trying to connect thirty lemon-cells in
CORRECTED: Ice skates do not function by melting ice via pressure
It is commonly stated that ice skates have low friction because ice melts when pressure is applied to it. This is not quite correct. A demonstration using an ice cube, a wire, and two weights is often provided to illustrate the phenomena. However, while pressure does affect the melting point of ice, the pressure provided by the skates is not enough to melt ice except when the temperature is a fraction of a degree below 0C. Also, the icecube and wire demonstration is very misleading because it is always performed in a heated room, and the wire doesn't melt ice entirely by pressure, it melts the ice by thermal conduction of warm room temperature along the wire. (Also, narrow gaps in ice always freeze closed because the simultaneous melt/freeze process at water/ice boundary acts to flatten points and fill crevices) Another point: the weight of small objects is too low to create high pressure, yet small objects do experience low friction when on ice. The low friction of ice is probably caused by a layer of liquid water a few hundred molecules thick which always spontaneously develops on the surface of ice. Also, melting from frictional heating can provide liquid water as lubrication. Here's more on this whole debate, and also a bit from BAD CHEMISTRY
CORRECTED: there are not 92 elements on Earth
Uranium has the highest atomic number of the elements commonly found in
the environment, and some books will tell you that there are 92 elements
found on earth: atomic numbers 1 through 92 (hydrogen
through uranium). This is wrong. Unfortunately there are two elements below Uranium
which are radioactive and have extremely short half lives. These are
Technetium and Promethium. These two elements do not occur naturally on
Earth, and this reduces the total number of elements found
in the environment to 90. However, in the 1970s a natural uranium reactor
was found in an ancient streambed in Africa, and the mineral deposits at
the site contained traces of a long-lived
Plutonium isotope (atomic number 94.) This brings the
total number of elements on the Earth back up to 91. (Note: Technetium,
though not found naturally on Earth, is present in some stars, detected by
spectral analysis.) See THE PHYSICS TEACHER, Vol.27 No.4 p282
Light from the sun is parallel? Nope.
Some books state that because the sun is so far away, sunlight arriving at the Earth is almost perfectly parallel. This is incorrect. The book authors reason that, the more distant the object, the more parallel the light, and since the sun is so far away, sunlight is perfectly parallel. They make a mistake. While it is true that light from *each tiny point* on the sun's surface is just about perfectly parallel by the time it reaches our eyes, light from the sun as a whole is not. This is because the sun, though very distant, is very large. A similar situation exists with light from the sky. We wouldn't say that the blue sky emits parallel light. Yet light from the sky comes from many miles away.
CORRECTED: with an aircraft wing, the lifting force does not come from the difference in curvature between the top and bottom surfaces.
First read the entire: wings/lift webpage
Some books say that the lifting force appears
because the wing's upper surface is longer than the
They state that air dividing at the leading edge of the wing must rejoin
at the trailing edge, therefore the upper air stream must move faster, and
so the wing is pulled upwards by the Bernoulli Effect. This is not
correct: the air divided by the leading edge
does NOT rejoin at the
trailing edge, and there is no "race" to catch up.
The same books often
contain a misleading diagram showing a
flat-bottomed wing with flow lines of the surrounding air. (see below.)
This diagram actually shows a zero-lift condition. The lifting force is
zero because the air behind the airfoil does not descend. In order to
lift in a three-dimensional situation, a wing must deflect air
Both the explanation and the diagram have serious problems. They wrongly
imply that inverted flight is impossible. They wrongly imply that an
aircraft with a symmetrical wing (a wing with equal
pathlengths above and below) will not fly. They also
wrongly suggest that an aircraft can violate the conservation of momentum
by remaining aloft without reacting against the air, and without causing a
downward motion of the air.
Yet upside-down flight is far from
impossible; it is a common aerobatic move. And many wings have equal
pathlengths, including even the thin cloth wings of the Wright Brothers'
flyer! And anyone standing under a slow, low-flying plane, or below the
thin, fast wings of a helicopter will know that there is a very great
downward flow of air below the wings. All of this indicates that there is
a serious problem with the "curved top, flat bottom" explanation. Below
is an alternative.
Go listen to NPR Science Friday Radio Archive, where physicist D. Anderson
debunks the various lifting-force myths.
Also see the NASA Aerodynamics Education site:
As a plane flies, the leading edges of its wings have little effect on the air, while the trailing edges have a huge effect. The wings' trailing edges always move through the air at an angle. This "effective angle of attack" causes the wing to apply a downward force to the air. In order to create lift, the wing must be tilted. Or rather than being tilted, the wings can be curved or "cambered", which makes the trailing edge of the wing tilt downward at an angle. The trailing edges of the wings cause the departing air to move downwards at an angle. As a result, the wing is pushed upwards and backwards. (These two pushes are called "lift" and "induced drag.")In other words, as any plane flies, its wings must send a stream of air diagonally downwards, and the wing acts like a 'reaction engine' just like a jet engine or a rocket. Unless a wing is either tilted or cambered, it cannot force the air downwards and cannot generate any "lift."
It may help to imagine a hovering
helicopter: a helicopter can hover because its rotor applies a downward
force to the air, and the air applies an upward force to the rotor. As a
result, the air flows downwards while the upward force supports the
But like any airplane, a helicopter rotor is a moving wing, and it's this
small tilted wing which
sends the air downwards. Like any wing, helicopter rotors are reaction
push air downwards, and the air pushes them upwards. They are not "sucked
upwards," and neither are airplanes.
You may have seen a plane's downwash of air in movies: a "cropduster"
plane sends out a trail of fertilizer mist, and the trail of mist does not
float, instead it moves immediately down into the crops, driven downward
by the moving air. Air from wings can even be dangerous: if a plane flies
too low, the downwash from its wings can knock people over.
The "Bernoulli effect" is still true. It explains how the top of the
wing is able to "pull downwards" on
the air flowing over it. And the Bernoulli Effect proves extremely useful
of the lifting force during classes in airplane physics and during
experimental work in aerodynamics. But airplanes also obey Newton's
laws: accelerate some air downwards, and you'll experience an upwards
Sound travels better through solids? No.
Many elementary textbooks say that sound travels better through solids and
liquids than through air, but they are incorrect. In fact, air, solids,
are nearly transparent to sound waves. Some authors use an experiment to
convince us differently: place a solid ruler so it touches both a ticking
watch and your ear, and the sound becomes louder. Doesn't this prove that
wood is better than air at conducting sound? Not really, because sound
has an interesting property not usually mentioned in the books:
waves of sound traveling inside a solid will bounce off the air outside
The experiment with the ruler merely proves that a wooden rod can act as a
sort of "tube," and it will guide sounds to your head which would
otherwise spread in all directions in the air. A hollow pipe can also be
used to guide the ticking sounds to your head, thus illustrating that air
is a good conductor after all. Sound in a solid has difficulty getting
past a crack in the solid, just as sound in the air has difficulty getting
past a wall. Solids, liquids, and air are nearly equal as sound
It's true that the speed of sound differs in each material,
but this does not affect how well they conduct. "Faster" doesn't mean
"better." It is true that their transparency is not exactly the same, but
this only is important when sound travels a relatively great distance
through each material. It's also true that complex combinations of
materials conduct sound differently and may act as sound absorbers
(examples: water with clouds of bubbles, mixtures of various solids, air
filled with rain or snow.) And last: when you strike one object with
sound created inside the solid object is louder than the sound created in
the surrounding air. So, before we try to prove that solids are better
conductors, we had better make sure that we aren't accidentally putting
louder sound into the solids in the first place.
Gravity in space is zero? Wrong.
Everyone knows that the gravity in outer space is zero. Everyone is
wrong. Gravity in space is not zero, it can actually be fairly strong.
Suppose you climbed to the top of a ladder that's about 300 miles tall.
You would be up in the vacuum of space, but you would not be weightless at
all. You'd only weigh about fifteen percent less than you do on the
ground. While 300 miles out in space, a 115lb person would weigh about
100lb. Yet a spacecraft can orbit 'weightlessly' at the height of your
ladder! While you're up there, you might see the Space Shuttle zip right
by you. The people inside it would seem as weightless as always. Yet on
your tall ladder, you'd feel nearly normal weight. What's going on?
The reason that the shuttle astronauts act weightless is that
they're inside a container which is falling! If the shuttle were to
sit unmoving on top of your ladder (it's a strong ladder,) the shuttle
would no longer be falling, and its occupants would feel nearly normal
weight. And if you were to leap from your ladder, you would feel just as
weightless as an astronaut (at least you'd feel weightless until you hit
So, if the orbiting shuttle is really falling, why doesn't it hit
the earth? It's because the shuttle is not only falling down, it is
moving very fast sideways as it falls, so it falls in a curve. It moves
so fast that the curved path of its fall is the same as the curve of the
earth, so the Shuttle falls and falls and never comes down. Gravity
strongly affects the astronauts in a spacecraft: the Earth is strongly
pulling on them so they fall towards it. But they are moving sideways so
fast that they continually miss the Earth. This process is called
"orbiting," and the proper word for the seeming lack of gravity is called
"Free Fall." You shouldn't say that astronauts are "weightless," because
if you do, then anyone and anything that is falling would also be
"weightless." When you jump out of an airplane, do you become weightless?
And if you drop a book, does gravity stop affecting it; should you say it
becomes weightless? If so, then why does it fall? If "weight" is the
force which pulls objects towards the Earth, then this force is still
there even when objects fall.
So, to experience genuine free fall just like the
astronauts, simply jump into the air! Better yet, jump off a diving board
at the pool, or bounce on a trampoline, or go skydiving. Bungee-jumpers
know what the astronauts experience.
Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. --Fred Hoyle
CORRECTED: For every action, there is not an equal and opposite reaction.
Newton originally published his laws of motion in Latin, and in the English translation, the word "action" was used in a different way than it's usually used today. It was not used to suggest motion. Instead it was used to mean "an acting upon." It was used in much the same way that the word "force" is used today. What Newton's third law of motion means is this:
For every "acting upon", there must be an equal "acting upon" in the opposite direction.Or in modern terms...
For every FORCE applied, there must be an equal FORCE in the opposite direction.So while it's true that a skateboard does fly backwards when the rider steps off it, these motions of "action" and "reaction" are not what Newton was investigating. Newton was actually referring to the fact that when you push on something, it pushes back upon you equally, even if it does not move. When a bowling ball pushes down on the Earth, the Earth pushes up on the bowling ball by the same amount. That is a good illustration of Newton's third Law. Newton's Third Law can be rewritten to say:
For every force there is an equal and opposite force.
CORRECTED: Ben Franklin's kite was never struck by lightning
Many people believe that Ben Franklin's kite was hit by a lightning
bolt, and this is how he proved that lightning is electrical. A number
of books and even some encyclopedias say the same thing. They are wrong.
When lightning strikes a kite, the electric current in the string is so
high that just the spreading electric currents in the ground can kill
anyone standing nearby, to say nothing of the person holding the string!
What Franklin actually did was to show that a kite would collect a tiny
bit of electrical charge-imbalance out of the sky during a thunderstorm.
Air is not a perfect insulator. The charges in a thunderstorm are
constantly leaking downwards through the air and into the ground.
Electric leakage through the air caused Franklin's kite and string to
become charged, and the hairs on the twine stood outwards. The twine was
then used to charge a metal key, and tiny sparks could then be drawn from
the key. Those tiny sparks were the only "lightning" in his experiment.
(He used a metal object because sparks cannot be directly drawn from
the twine; it's conductive, but not conductive enough to make sparks.)
His experiment told Franklin that some stormclouds carry strong electrical
charges, and it implied that lightning was just a large electric
The common belief that Franklin easily
survived a lightning strike is not just wrong, it is dangerous: it may
convince kids that it's OK to duplicate the kite experiment as long as
they "protect" themselves by holding a silk ribbon and employing a metal
key. Make no mistake, Franklin's experiment was extremely dangerous.
Lightning goes through miles of insulating air, and will not be stopped by
a piece of ribbon. If lightning had actually hit his kite, he would have
been gravely injured, and most
probably would have died instantly. See LIGHTNING SURVIVOR RESOURCES
The main lens of your eye is inside the eye? Not quite.
Some textbooks assume that the small lens found deep within the eyeball is the
eye's main lens, and the cornea of the eye is simply a protective window.
The textbook diagrams even depict light rays passing into the eye and only
bending as they pass through this internal lens. But in the human eye,
the small lens found within the eyeball is not the main imaging lens. The
cornea is actually the main lens; it is the strongly curved transparent
front surface of the eye. Most of the bending of the light occurs at the
place where the light enters the surface of the cornea. When you look at
your eye in the mirror, you are looking directly at the eye's main lens.
When you want to change the focusing power of your eye, you apply
"contact lenses" to the cornea surface, or you undergo surgery which
re-sculpts the cornea's curvature. The smaller lens inside the eye acts
only to alter the focus of the eye as a whole. Muscles change its shape
in order to correct the focus for near and far viewing. Without this
small internal lens, human vision would be blurry, and vision
unable to accommodate for near and far views. But without the cornea lens,
[the human eye would be blind.] IMPROVED VERSION: without the
vision would rely upon the pinhole-camera effect of the eye's pupil,
and vision would be incredibly blurry. Open your eyes underwater in
dimly-lit conditions to see what vision would be like without a cornea.
CORRECTED: when one prism splits light into colors, a second identical prism cannot recombine them.
A single prism can split a sunbeam into a rainbow. Many children's
science books show how a second similar prism can be used to recombine the
colors. This is incorrect, two prisms do not work as shown. Prisms of two
different sizes can split and then focus the colors into momentary
recombination at a particular distance. With three prisms in a
special arrangement, the splitting and complete recombining of colors can
be accomplished. But books which depict one prism splitting the colors
and a second identical prism recombining the colors into a single white
beam are in error, and are no doubt the source of endless frustration for
those of us who try to duplicate the effect with real prisms.
The "rainbows" can also be recombined by placing a screen at just
the right place, and by bouncing the colors off many small mirrors so the
colored beams converge upon a screen. Recombination can also be done with
a convex lens or a concave mirror and a screen. I hope that very few
students will attempt to perform the color recombination experiment
depicted in their books, for disappointment awaits. (MORE)
Clouds, fog, and shower-room mist are water vapor? No.
All three things are made of small droplets of liquid water hanging in
the air. When water
it turns into a transparent gas called "water vapor." When it condenses
again, it can take the form of rain, snow, rivers, and oceans, but it also
can take the
form of clouds, mist, fog, etc. Fog can make surfaces wet, but not
because of condensation. Instead, the fog droplets collide with the
solid surface. Fog is liquid water, not a vapor. Fly an ultralight
slowly through a large dense cloud, and you'll become damp. To look for
water vapor, look at the bubbles in rapidly boiling water. Look at the
small empty space at the spout of a boiling teakettle. Look at the far
end of the teakettle's plume of mist, where the mist seems to vanish into
the air. Look at the empty air above a wet surface. In these situations
you see nothing, and that's where the vapor is. Water vapor seems
invisible because it is transparent. Clouds and fog are not transparent.
They are composed of liquid droplets.
CORRECTED: raindrops don't have points!!
drawing of raindrops depicts them as having a sharp upper point. This is
wrong. Surface tension of water acts like a stretched "bag" around the
water, and unless some other force is acting, it pulls the water into a
spherical shape. Our eyes do see tiny droplets as a blur, but a flash
photograph reveals that small raindrops are nearly spherical. The larger
ones are distorted by the pressure of moving air, but this doesn't make
points, it makes them somewhat flattened. Think of it this way:
underwater bubbles are not pointed as they rise, just as falling water
drops are not pointed as they fall. And while it's true that the
for water is a droplet with a point, real water droplets look
the symbol. And when water drips from a faucet, it never actually has a
point. Instead it has a narrow neck, and after the neck has snapped, it
is yanked back into the falling ball of water. See Dr. Fraser's BAD SCIENCE for
lots more about this.
Air is weightless? No.
We are not conscious of
air's weight because we are immersed within it. In the same way, even a
large bag of water seems weightless when it is immersed in a water tank.
The bag of water in the tank is
supported by buoyancy. In a similar way,
buoyancy from the atmosphere makes a bag of air seem weightless when it's
surrounded by air. One way to discover the real weight of air would be to
take a bag of air into a vacuum chamber. Another way is to weigh a
pressurized and an unpressurized football. A cubic meter of air at
sea-level pressure and
0C temperature has a mass of 1.2KG. The non-metric rule of thumb says
that the air that would fill a bathtub weighs about one pound.
Here's a simple way to
the mass of air even though the air seems weightless: open an umbrella,
wiggle it slightly forwards and back, then close it and wiggle it again.
When you wiggle it when open, you can feel its increased mass because of
the air the umbrella must carry with it. (Ah, but then we must explain
the difference between weight and mass!)
CORRECTED: Filled and empty balloons do not demonstrate the weight of air.
Many books contain an incorrect experiment which purports to directly
demonstrate that air has weight. A crude beam-balance is constructed
using a meter stick. Deflated rubber balloons are attached to the ends,
and the balance is adjusted. One balloon is then inflated, and that end
of the balance-beam is supposed to sag downwards. The demonstrator then
explains that a large amount of air weighs more than a small amount of
Unfortunately this experiment isn't very honest. When immersed in
causes full and empty balloons to weigh the same. One balloon shouldn't
pull down the stick. But then why does the
above experiment work? Usually it doesn't! In fact, the experiment will
fail unless you know the trick: you must inflate the balloon near to
bursting. The experiment secretly relies on the fact that the air within
a high-pressure balloon is denser than air within a low pressure balloon.
Of course the demonstrator never mentions this to the students, and the
books which contain this demonstration don't mention density effects
either. Obviously the density effects do not directly demonstration
anything about the weight of air, so it's dishonest to tell students that
this demonstration can directly weigh some air.
To illustrate the problem, try this instead: attach two opened paper bags
to the balance, adjust it, then crush one bag so it contains little air.
The balance will not change. What does this teach your class; that
air is... weightless? Yet the air does have significant weight. We just
can't detect this weight directly by using paper bags. Or using
Here's a way to make the experiment more honest. Perform the balance-beam
experiment again, but use two full balloons. Blow one balloon
really full so the rubber feels hard and the balloon is about to
pop. Blow up the second balloon so it is almost full, but still a
stretchy. Try to keep the balloons almost the same size. Now the balance
will show that, even though the balloons look nearly the same, the "hard"
balloon is significantly heavier. Does this teach misleading things to
your class? Not really, instead it exposes the dishonesty of the original
demonstration. In truth, balloons filled with air will not weigh more
than empty balloons as long as they remain immersed in the atmosphere.
However, compressed air does weigh more than uncompressed
air. Perhaps this modified demonstration would be appropriate in more
advanced classes. But this website is about K-6 grade science.
Here's another way to think about it. Can we demonstrate the weight of
water to a fish?
What if we lived underwater, how could we use the balance-beam to measure
the weight of water directly? The answer is that we cannot. If a
water-filled balloon and a collapsed empty balloon were compared
the experiment would show that they weigh the same, which seems to prove
that water is weightless. When underwater, a bag full of water weighs
just the same as a flattened bag which contains nothing. The situation
with air is similar: if we live our lives immersed within a sea of air,
we cannot use a balance to easily detect the actual weight of the air. (In
fact, a bathtub full of air weighs about a half kilogram, but we cannot
easily demonstrate this weight while living in an atmosphere.)
It's hard to teach the weight of water to the fishes, and hard to teach
the weight of air to human grade-schoolers. These misleading textbook
experiments could only work correctly if performed in a vacuum environment
(say, on the moon's surface.) We humans are like fish underwater: we're
not aware that our ocean of air has any weight. To demonstrate the
weight, we need to get out into a vacuum environment.
Or, to better demonstrate the weight of air directly, hook a heavy bottle to a vacuum pump, pump all the air out, seal it, then weigh the bottle. Break the seal and let the air in, then weigh it again. The difference in weight is the weight of the air contained in the bottle. Another: use a balance to compare the weight of two vacuum-containing bottles, then open one of them so it becomes filled with air. The bottles will then weigh differently, and the difference is the true weight of the air in one bottle. Or another: build a balance using upside-down paper bags, then place a candle below one of them, then remove the candle again. That bag rises, indicating that a volume of warm air weighs slightly less than a volume of cool air. (Don't set the bag on fire!!) But note that this candle experiment is a bit like the compressed balloon version, and it says nothing simple and direct about the actual weight of a volume of unheated air.
CORRECTED: in the everyday world, gases do not expand to fill their containers.
What is the difference between a liquid and a gas? Both are "fluids",
both can flow. Gases are usually less dense than liquids, although
gases under fiercely high pressure can approach the density of liquids, so
that's not a good criterion. The main difference is that gases are a
different phase of matter: a gas can be made to condense into a liquid
form, and a liquid can be made to evaporate into gas. Another major
characteristic: because there are bonds between its particles, when a
liquid is placed into a vacuum environment, it will not immediately
and continuously expand, while a gas in a vacuum chamber will expand at
high velocity until it hits the container walls.
This is very different from the oft-quoted rule that "gases always expand
to fill their containers." This rule only works correctly if the
container is totally empty: the container must "contain" a good
vacuum beforehand. However, we all live in a gas-filled environment. All
our containers are pre-filled with air. In our environment, any new
quantity of gas will not expand, it will just sit there. It may slowly
diffuse outwards, but that's very different than the "expansion" being
discussed here. If
some carbon dioxide out of a CO2 fire extinguisher, it will not instantly
expand to fill the room. Instead it will pour downwards like an invisible
fluid and form a pool on the
floor. It behaves similarly to dense sugar-water which was injected into
a tank of water: it pours downwards, and only after a very long time it
will mix with the rest of the water. "Mixing" is very different from
"expanding to fill!" The rule about gases does not involve mixing;
instead it involves compressibility and instant expansion into a vacuum.
In an air-filled room, dense gases act much like liquids; they can be poured into a cup or bowl, poured out onto a tabletop, and then they run off the edge onto the floor where they form an invisible mess. :) Less dense gases will stay where they are put, like smoke or like food coloring which has just been injected into a fishtank. Gas of even lesser density rises and forms a pool on the ceiling. Only in the world of the physicist, where "empty container" always implies a vacuum, does the rule about gasses work properly.
CORRECTED: Shadows do vanish on cloudy days, but not because the sun isn't bright enough.
Shadows appear when an object blocks a light source. The shape of the
shadow is created by the shape of the opaque object and by the
shape of the light source. On a cloudy day the whole sky acts as a light
source, and a person's shadow spreads out and becomes a dim fuzzy patch
which surrounds the person on the ground on all sides. The shadow is so
spread-out that it seems absent entirely. When the sun is visible, the
same shadow is concentrated in one specific place and becomes easy to see.
But even the shadows made by sunlight will have fuzzy borders, since the
sun is a small disk rather than a tiny dot. On cloudy days, the fuzzy
borders of your body's shadow become much much larger than the shadow
itself, so that the shadow seems to vanish.
CORRECTED: FRICTION IS NOT CAUSED BY SURFACE ROUGHNESS
Some books point to surface roughness as the explanation of sliding
friction. Surface roughness merely makes the moving surfaces bounce up
and down as they move, and any energy lost in pushing the surfaces apart
is regained when they fall together again. Friction is mostly caused by
chemical bonding between the moving surfaces; it is caused by stickiness.
Even scientists once believed this misconception, and they explained
friction as being caused by "interlocking asperites", the "asperites"
being microscopic bumps on surfaces. But the modern sciences of surfaces,
of abrasion, and of lubrication explain sliding friction in terms of
chemical bonding and "stick & slip" processes. The subject is still full
of unknowns, and new discoveries await those who make surface science
When thinking about friction, don't think about grains of sand on
sandpaper. Instead think about sticky adhesive tape being dragged along a
CORRECTED: NO, INFRARED LIGHT IS NOT A KIND OF HEAT
Infrared light is invisible light.
When any type of light is absorbed by an object, that object will be
heated. The infrared light from an electric heater feels hot for two
reasons: because surfaces seem black to IR light, and because the IR is
extremely bright light. Just because human eyes cannot see the
causes the heating does not mean it's made of some mysterious entity
called "heat radiation." When bright light shines on an absorptive
surface, that surface heats up.
And this is no benign misconception. Those who fall under its sway may
also come to believe that *visible* light cannot heat surfaces (after all,
visible light is not "heat radiation?") Misguided science students may
wrongly believe that warm objects emit no microwaves (since only IR light
is "heat radiation"), even though hot objects actually do emit microwaves.
Or students may believe that the glow of red hot objects is somehow
different from the infrared glow of cooler objects. Or they may believe
that IR light is a form of "heat," and is therefore fundamentally
different from any other type of electromagnetic radiation.
In his book "
Clouds in a Glass of Beer," Physicist C. Bohren points out that this
"heat" misconception may have been started long ago, when early physicists
believed in the existence of three separate types of radiation: heat
radiation, light, and actinic radiation. Eventually they discovered that
all three were actually the same stuff: light. "Heat radiation" and
"actinic radiation" are simply invisible light of various frequencies.
Today we say "UV light" rather than "actinic radiation." Yet the obsolete
term "heat radiation" still lingers. Since human beings can only see
certain frequencies of light, it's easy to see how this sort of confusion
got started. Invisible light seems bizarre and mysterious when compared
to visible light. But "invisibility" is caused by the human eye, and is
not a property carried by the light. If humans could see all the light in
the infrared spectrum, we would say things like this: "of course
electric heater makes things hot at a distance, it is intensely
and bright light can heat up any surface which absorbs it."
PS, if you're interested in physical science misconceptions, Bohren's Book is an excellent resource. He's like me, and complains about several specific misconceptions which keep his students from understanding science.
CORRECTED: THERE ARE NOT SEVEN COLORS IN THE RAINBOW
Actually there's a very large number of distinct colors in any rainbow.
And neither are there sharp divisions between the bands of color, yet
numerous textbooks depict them. In reality, between yellow and green we
find yellow-green, and between green and yellowgreen is greenish
yellowgreen, and on and on. How many colors are in a rainbow? Thirty?
Sixty? It's not easy to say, for it depends on the particular eye, and
the particular rainbow. What of the teachers and students who look in
vain for the yellow-green in their textbook's depiction of rainbows?
They've crashed into a long-running textbook misconception: the strange
idea that rainbows have exactly seven distinct bands of color and no more,
and with nothing in between those uniform bands of 'official' color.
CORRECTED: ACTUALLY, THE EARTH'S NORTH AND SOUTH MAGNETIC POLES RESIDE DEEP WITHIN THE EARTH'S CORE
Many textbooks have an erroneous diagram of the earth which shows
a bar magnet within it, and the ends of this bar magnet extend to just
beneath the earth's surface. These diagrams depict the magnet's field
lines as radiating from spots on the earth's surface. This is very
misleading. The earth's magnetic poles actually behave as if they're deep
within the earth, down inside the core. The Earth's magnetic field does
not come from a giant bar magnet, but if we imagine that it does,
then the imaginary "bar magnet" inside the earth is short, stubby,
disk-shaped, and part of the iron core deep inside the planet.
The typical textbook diagram is incorrect, and there are no intense
magnetic fields at the land surface near the earth's "north pole" and
"south pole." If you stand at the Earth's south magnetic pole, metals
aren't attracted to the ground more strongly than anywhere else. The
Geomagnetic "poles" on the earth's surface are not places where the field
is strong. They are simply the points on the landscape where the field
lines are perfectly vertical.
should instead show the field lines to be radiating from poles inside the
earth's core. They should show the field lines around the northern and
southern areas of the earth's surface as being approximately vertical and
parallel, not "radial" like a spiderweb and not concentrated into special
points on the surface.
Another error associated with the above: some books claim that the
earth's field at the magnetic poles is much stronger than elsewhere.
This is untrue. The field strength at the north magnetic pole above
Canada is about the same as the field strength in Virginia! And the
strongest field in the Earth's northern hemisphere does not appear at the
north magnetic pole at all, the North Pole actually has a weaker field
than elsewhere. The strongest fields in the northern hemisphere are not
in one but in two places: west of Hudson Bay in Canada, and in Siberia.
LASER LIGHT IS "IN PHASE" LIGHT? WRONG.
It's incorrect to say that "in laser light the waves are all in phase."
When two light waves traveling in the same direction combine, they
inextricably add together, they do not travel as two independent
"in-phase" waves. The photons in laser light are in phase, but the WAVES
are not. Instead, ideal laser light acts like a single, perfect wave.
When the light wave within a laser causes atoms to emit smaller, in
phase light waves, the result is not "in phase" light. Instead the result
is a single, more intense, amplified wave of light. In-phase emission
leads to amplification, not to multiple in-phase waves. If the atoms'
emissions weren't in phase, the result would not be light that's
out of phase. Instead the atoms would absorb light rather than amplifying
Each atom in a laser contributes a tiny bit of light, but their light
vanishes into the main traveling wave. The light from each atom
strengthens the main beam, but loses its individuality in the process.
99 plus 1 equals 100, but if someone gives us 100, we cannot know if it is
made from 99 plus 1, or 98 plus 2, or 50 plus 50, etc.
Yes, it's true that all the photons associated with a single wave
of light are in phase. This might be one reason that people say that
laser light is "in phase" light. However, in-phase photons are nothing
unique, and they don't really explain coherence. Any EM sphere-wave or
plane-wave is made of in-phase photons. For example, all the photons
radiated from a radio broadcast antenna are also in phase, but we don't
say that these are special "in phase" radio waves, instead we just say
that they are waves with a spherical wavefront. Even if all the photons
in laser light are in phase, it is still incorrect to say "all the
WAVES are in phase." Photons are not waves. They are quanta, they
are particles, and they do not behave as small, individual "waves." Yes,
all the photons are in phase, but only because they are part of a single
The light from a laser is basically a single, very powerful light wave.
Single waves are always in phase with themselves, but it's misleading to
imply that a single plane-wave or sphere-wave is something called an "in
phase" wave. Laser light could more accurately be called "pointsource"
light. Sphere waves or plane waves behave as if they were emitted from a
single tiny point. The physics term for this is "spatially coherent"
light. Light from light bulbs, flames, the sun, etc. are the opposite,
and are called "extended-source" light. Extended-source light comes from
a wide source, not from a point-source, and the waves coming from
different parts of the source will cross each other. Starlight and the
light from arc welders is "point-source" light and is quite similar to
laser light. Light from arc-welders and from distant stars has a higher
spatial coherence than light from most everyday light sources. (Note: the
sun is a star, correctly implying that light becomes more and more
spatially coherent as it moves far from its source. This is a clue as to
the real reason that lasers give spatially coherent light! (See
CORRECTED: LASER LIGHT IS NOT PARALLEL LIGHT
Light from most lasers is not parallel light. However, if laser light is
passed through the correct lenses, it can be formed into a tight, parallel
beam. The same is not true for light from an ordinary light bulb. If
light from a light bulb were passed through the same lenses, it would form
a spreading beam, and an image of the lightbulb would be projected into
the distance. Laser light can form beams because a laser is a pointsource,
and when you project the image of a pointsource into the distance, you
form a narrow parallel beam! However, it is simply wrong to state that
laser light is inherently parallel light. Laser light can be
formed into parallel light, while the light from ordinary sources cannot.
Most types of lasers actually emit spreading, non-parallel light. Lasers
in CD players and in "laser pointers" are semiconductor diode lasers.
They create cone-shaped light beams, and if a parallel beam is desired,
they require a focusing lens. The same is true for the lasers in
inexpensive "laser pointers." Take apart an old laser-pointer, and you'll
find the plastic lens in front of the diode laser inside.
Classroom "HeNe" lasers also create spreading light. The laser tube
within a typical classroom laser contains at least one curved mirror
(called a "confocal" arrangement,) and it creates light in the form of a
spreading cone. It's a little-known fact that manufacturers of classroom
lasers traditionally place a convex lens on the end of their laser tubes
in order to shape the spreading light into a parallel beam. While it's
true that a narrow beam is convenient, I suspect that part of their reason
is to force the laser to fit our stereotype that all lasers produce thin,
narrow light beams. The manufacturers could save money by selling "real"
lensless laser tubes having spreading beams. But customers would
complain, wouldn't they? We have been brought up to believe that laser
light is parallel light.
CORRECTED: LASERS EMIT COHERENT LIGHT, BUT NOT BECAUSE THE ATOMS EMIT IN-PHASE LIGHT WAVES
In-phase emission causes the amplification of light, it doesn't
cause coherent light. Because the atoms emit light in phase with incoming
light, they will amplify the light, but they amplify incoherent light too,
and they don't make it coherent. The coherence of laser light has another
Laser light has two main characteristics: it is "monochromatic" or very
pure in frequency (this also is called "temporally coherent.") Laser
light also has a point-source character of sphere waves and plane waves
(also called "spatially coherent.")
Even fairly advanced textbooks fail to give the real reason why laser
light is spatially coherent. They usually point out that the laser's
atoms all emit their light in phase, and pretend that this leads to
spatial coherence. Wrong. It is true that the fluorescing atoms in a
laser all emit light that's in-phase with the waves already traveling
between the mirrors. But the in-phase emission only creates
amplification of the traveling waves, it does not create spatially
coherent light. For example, if you were to feed incoherent light
into a HeNe laser tube, the atoms would emit in-phase waves, and the laser
would amplify the light. But the brighter light would still be
incoherent! Lasers certainly can amplify the coherent wave which
is trapped between their mirrors. But how did the light within the laser
get to be coherent in the first place?
Lasers create coherent light because of their mirrors.
The mirrors in a laser form a resonant cavity which preserves coherent
light while rejecting incoherent light. How does it work? Imagine a
simplified laser having flat, parallel mirrors. As light bounces between
the mirrors, the light "thinks" that it's traveling down an infinitely
long virtual tunnel. (Have you ever held up two mirrors facing each
other? Then you've seen this infinite tunnel.) When a laser is first
turned on, it fluoresces; it emits light which is not coherent.
Different random light waves start out from different parts of the laser.
After a few thousand mirror bounces, all the waves have added and
subtracted to form just one single EM wave. In the case of flat-mirror
lasers, this wave is a nearly perfect plane wave. A single plane wave is
coherent (to be incoherent, you must have at least two different
This can be a bit confusing. After all, the individual atoms each emit a
wave. Don't all these waves add up to messy incoherent light? No. The
in-phase emission preserves the existing coherence as it amplifies. It's
true that each atom emits light waves in all directions. However, these
sideways waves from all the atoms will cancel each other out, and only the
waves that travel in the same direction as the incoming light will be
preserved. It's as if the atoms "know" which direction to send out their
beam. But in reality, the atoms don't need to know the beam direction.
Instead, they just emit a light wave which is in phase with the incoming
light, and for this reason the wave from the atom will cancel out
everywhere except in a line with the incoming light. If the light in a
laser were already coherent, then the atoms will amplify it but
won't make it more coherent. The coherence comes from the great distance
that the light has traveled as it bounced between the mirrors.
A similar thing happens with starlight: starlight is coherent! Starlight
travels far from its original source and all the waves from different
parts of the star will add up to form a wave with a single planar
wavefront. Light from distant stars is spatially coherent, even though
sunlight is not, yet the sun is a star too. The farther the light travels
from its source, the more it approaches the shape of a perfect plane wave.
And a perfect plane wave is perfectly coherent. Laser light is spatially
coherent because, among other things, the bouncing light has traveled
millions of miles between mirrors, and all the various competing waves
have melded together to form a single pure plane-wave or
P.S. The pure color (monochrome) laser light is also
created by the
mirrors. Huh? Yes, but the reason for this is not totally
straightforward (and it's quite a bit beyond the K-6 level of these
The two mirrors of a laser can trap a standing wave of light. The space
between the mirrors is like the string of a guitar: there can be a
fundamental wave, or overtone waves, or complicated waves which are a
mixture of these. But waves of non-overtone frequencies cannot exist
between the mirrors. Since the distance between the crests of a light
wave is very small, lots of different overtones can fit between the
mirrors, and each overtone is a slightly-different pure color of light.
Light from a neon sign is reddish, but it doesn't have the extreme purity
of laser light. Now for the weird part: when a Helium-Neon laser first
operates, many different overtones of red light are amplified and the beam
contains many slightly-different colors of red at the same time. It's not
yet monochromatic. As time goes on, some of these colors are amplified a
bit more than others, and this uses up the available energy coming from
the laser power supply. In other words, the different waves start
competing for limited resources! Just one wave "wins" in the end, and all
of the other overtones drop out of the running. The laser light is not
just red light. Instead it is a single pure overtone-wave, a pure
frequency where the string of waves just perfectly fits in the space
between the two mirrors. Change the spacing of the laser's mirrors (for
example by heating a glass HeNe laser tube,) and you change the frequency
of the light.
CORRECTED: IRON AND STEEL ARE NOT THE ONLY STRONGLY MAGNETIC MATERIALS
There are numerous others. Nickel and
metals are very magnetic. (U.S. "nickel" coins contain copper which
spoils the effect, so try Canadian nickels made before 1985.) Most other
materials are "diamagnetic," and are repelled visibly by very strong
magnets, although some materials are "paramagnetic" and are attracted.
Supercold liquid oxygen is attracted by magnets. Some but not all types
of stainless steel are nonmagnetic. There are even some metals which are
individually nonmagnetic, but which become strongly magnetic when mixed
together, chromium and platinum for example, and compounds of manganese
CORRECTED: RE-ENTERING SPACE CAPSULES ARE NOT HEATED BY AIR FRICTION
They are heated as they plow into the atmosphere and compress the air ahead of them. Ever pump up a bicycle tire and discover that the pump and the tire have become hot? The same effect causes spacecraft and supersonic aircraft to heat up as they compress the air at their leading edges. The heat doesn't come from *rubbing* upon the air, it comes from *squeezing* the air. This applies mostly to blunt objects such as Apollo reentry vehicles. It does not apply as much to the Space Shuttle: with wings oriented mostly edge-on to the moving air, the surfaces of the Shuttle are heated by friction. But when the Shuttle first reenters the atmosphere, the bottom of the craft faces forwards, and in that case the Shuttle is heated by air compression, not by friction.
CORRECTED: CARS AND AIRPLANES ARE NOT SLOWED DOWN BY AIR FRICTION
They are slowed because it takes energy to stir the air. While
friction between the air and the car's surface does play a part, the work
done in stirring the air far exceeds the work done in direct frictional
heating. If vehicles did not send air swirls and vortices spinning off as
they moved, they would barely be slowed by the air at all. Eventually the
swirling air is slowed by friction and ends up warmer, but this occurs
long after the vehicle has passed.
CORRECTED: THE NORTH MAGNETIC POLE OF THE EARTH IS NOT IN THE NORTH
Opposite poles attract. If we hold two bar magnets near each
the "N" pole of one magnet is attracted by the "S" pole of another. If we
suspend a bar magnet by a thread, the "N" pole of that magnet will
point... toward the Earth's north!
Something is wrong here.
"N" pole of a magnet point towards the "S" of the Earth? Alike poles
should repel, not attract. Either the "N" and "S" printed on all bar
reversed, or the "N" and "S" on the Earth is backwards. Which is
This problem has a simple solution. Physicists define "N-type" magnetic
poles as being the north-pointing ends of compasses and magnets. This
definition is built into all of modern science and engineering and is part
of Maxwell's equations. Wind an electromagnet coil, see which end points
towards the Earth's North Pole, and that end is the "N pole" of the
electromagnet. And this means that the magnetic pole found deep inside
the northern hemisphere of the Earth is a south-type magnetic pole. The
Earth's northern magnetic pole is an S! It has to be this way, otherwise
it would not attract the N-pole of a compass.
This is a long-standing but arbitrary physical standard, much the same as
defining electrons as being negative. Like it or not, we are stuck with
negative electrons, with seconds which last about 1/100,000 of a day, with
backwards Earth poles, with centimeters which are about as wide as a small
CORRECTED: ACTUALLY THERE ARE NO SODIUM CHLORIDE MOLECULES IN SALT WATER
Salt is not made of NaCl molecules. Salt is made of a three-dimensional checkerboard of oppositely charged atoms of sodium and chlorine. A salt crystal is like a single gigantic molecule of ClNaClNaClNaClNaClNaClNa. When salt dissolves, it turns into independent atoms. Salt water is not full of "sodium chloride." Instead it is full of sodium and chlorine! The atoms are not poisonous and reactive like sodium metal and chlorine gas because they are electrically charged atoms called "ions." The sodium atoms are missing their outer electron. Because of this, the remaining electrons behave as a filled electron shell, so they cannot easily react and form chemical bonds with other atoms except by electrical attraction. The chlorine has one extra electron and its outer electron shell is complete, so like sodium it too cannot bond with other atoms. These oppositely charged atoms can attract each other and form a salt crystal, but when that crystal dissolves in water, the electrified atoms are pulled away from each other as the water molecules surround them, and they float through the water separately.
CORRECTED: LIGHT AND RADIO WAVES DO NOT ALWAYS TRAVEL AT "THE SPEED OF LIGHT"
They only travel at the "speed of light" (186,000 miles per second) while moving through a perfect vacuum. Light waves travel a bit slower in the air, and they travel lots slower when moving through glass. Why does light bend when it enters glass at an angle? Because the waves SLOW DOWN. Why can a prism split white light into a spectrum? Because within the glass the different wavelengths of light waves have different speeds And while the numerical value for the speed of light in a vacuum, "c," is very important in all facets of physics, as far as light waves are concerned there is no single unique speed called "The Speed Of Light." [note for advanced students: ok ok, I'll add this: light *waves* within a transparent medium are slow, even though the wave's photons are thought to jump from atom to atom always at a speed of c. But such ideas are not very honest, since whenever we only pay attention to the vacuum between particles in a solid, we stop treating the solid as a "uniform transparent medium." ]