Explanation and demonstrations.
(c)1996 William J. Beaty
Sheets of any transparent material will reflect light. If the
transparent sheet is nearly as thin as the wavelength of the light
waves, you'll get some strange effects which lead to visible colors.
Films of oil on water, soap bubbles, thin layers of air trapped between
glass or plastic sheets, very thin blown-glass balls, or even layers of
water drying on a polished dark surface, all of these display "oilfilm
colors" when illuminated with white light.
Since waves of visible light are around .5 millionths of a meter apart,
and there are about 40 inches in a meter, light waves are around 50,000
waves per inch. So, if your thin transparent layer is somewhat less
than 1/10,000 of an inch thick, you'll see colors.
The colors appear because of wave interference. The front of the thin
layer reflects light. So does the back surface of the layer. If the
layer is very thin, then the light waves reflected from the back and
from the front will add or subtract depending upon layer thickness and
upon the light wavelength. If broad-spectrum illumination is used, such
as white light bounced off a sheet of paper, an interesting rainbow will
be seen which contains additive and subtractive colors (not just the
pure-color spectrum of red/orange/yellow... spectrum, but also cyan,
An oilfilm colors trick: get a black bowl, or put some dark paper in a
bowl of water to give a black background. Next, put the bowl in the
sink, fill it with wather, then let a thin stream of water run into the
bowl so it overflows. To prevent ripples and splashing, you may need to
sit the bowl on something tall so it is lifted up near the spigot, so a
thin silent stream of water keeps overfilling the bowl. Make sure
that the room lights are fairly bright so the ceiling is well lit. Now
drip various kinds of oil onto the water surface. You'll see explosions
of oilfilm colors, and the overflow will suck them away so you can drip
more oil. To avoid making a big greasy mess in the sink, only use the
tiniest drops. Try dipping a corner of a piece of paper in the oil and
then touching it to the water. Try dripping two oildrops and watch the
bursts of color repel each other. Float some toothpicks to form walls,
and see how they affect the oil flow. Make floating squares from
toothpicks, fill the inside with an oil drop, then move them apart to
let out the colors.
(Safety note: young children shouldn't play with rubbing alcohol except
under close supervision. It's dangerous if they drink it, get it in eyes,
etc. Also, alcohol is flammable, so no smoking if you do these demon-
strations!) Make some soapbubbles and hold them up against a dark
background so their colors are easily seen. Dip a cotton swab in some
rubbing alcohol and wave it near the bubble. The alcohol vapor will
change the surface-tension of the bubble, which sucks more water to some
parts and pulls it away from others, which makes the water film
thickness change, and makes the colors roil! This is more easily seen
if you make a large ring from heavy wire or coathanger, dip it in bubble
solution, observe it against the dark background, then let the alcohol
vapor hit the soap film. Or roll a little tube of paper, dip one end in
alcohol, then blow through it gently towards the bubble. Blow the
alcohol fumes ACROSS your large bubble, and you'll almost be able to
SEE the stream of alcohol gas.
Still more: blow some bubbles outside at night under mercury-vapor
streetlights (the blue-white kind) or near a bright red neon sign.
The colors will be very strange compared to whitelight oilfilm colors.
ENVIRONMENTAL NOTE: It's bad to pour large quantites of oil down
the drain, it will find it's way to lakes or oceans and cause water
pollution. Sunlight will destroy the few drops of oil you wash
down the drain during this experiment, so don't worry about it.
But stay aware of oil pollution problems. If you change your car's
oil, take it to a gas station for disposal, and if your car has
an oil leak, get it fixed!