William Beaty 2/24/95

I've always been fascinated with "heat radiation;" otherwise known as longwave infrared light. You don't need a military night-vision camera, since there is an inexpensive imaging detector for this: liquid crystal postcards. These postcards are available from several science supply catalogs, as well as local museum stores, also some mall gift shops, etc. If you heat the postcard until it just begins to change color, it becomes sensitive to small amounts of IR light, and acts like instant camera film. Some things to try:

Shine a hot (100 watt) light bulb on the IR postcard, and make a shadow with your hand. A thermal shadow appears on the postcard. Still shining the light, hold a piece of flat glass in your hand so your hand and the glass cast a shadow. Although the shadow of the glass is invisible (except for the edge), the color-thermal shadow indicates an opaque object! Glass is transparent to visible, but acts "black" to long IR.

The opposite of the above demo can be performed with black plastic IR filter: when the black material is held up to cast a shadow, the colored thermal shadow will not appear (except for maybe the edge of the filter). Deep blue filter plastic will do this, and is even better when held together with a red filter to create visible-black, but IR transparent filter. Fully-exposed B&W film negatives also act like IR filters, as also do the silicon wafers available from some science suppliers.

Glass isn't totally opaque to long IR, so it is possible to use a lens to focus a hotspot on the postcard. The hotspot is actually an image of the lightbulb. Try doing the same with an electric heater! And try doing the same with the lens covered with one of the black filters above. Also, I think acrylic lenses and acrylic fresnel lenses might work even better, or the polyethelene fresnel lenses used on motion sensors for security lights.

I've never tried it, but it MIGHT be possible to pre-heat the postcard, then use a lens to cast the IR image of the view outside a window onto the card, so for a moment you would see an IR image of the world.

Use an ice cold cola can to make a cold ring-pattern on your arm, then quickly hold the LC postcard against your arm. The cold ring becomes visible. Look closely and you can even see veins and arteries for a moment as the LC postcard colors pass through their most sensitive range.

Cool some pennies, and heat some others in your hand. Scatter them on the LC postcard, then move them a bit. Some have colored shadows behind them, others do not. Or sprinkle some drops of hot and cold water onto the postcard and note the difference.

Use a pencil eraser to draw a "warm line" on a piece of wood, then hold the LC postcard in contact to expose the pattern. If you work fast enough, you can even write your initials in this way.

Point your finger upwards, then hold the LC postcard right above it. A plume of warm air wafts upwards along your finger, and after awhile the LC postcard will show its shape.

I found an article which mentions where large sheets of the material can be obtained:


    On other topics: have you ever heard of the lab done by an instructor years ago in THE SCIENCE TEACHER, where he had his students measure outdoor temperatures every few hours during the winter, in particular taking many measurements when the temperature was around 0C? They plotted temperature versus time as the temperature varied across 0C, and found a peak in the curve! It seems that as the temperature decreases, when it hits 0C it stops falling while environmental water freezes, then it continues downwards again. A similar thing happened when the temperature was rising past 0C. It wasn't a large effect, but the students were able to see it in their graphs. The instructor guided his students in coming up with sensible explanations for the existance of this "stick point" in the graph of outdoor temperature versus time.

    I recall running across an entry on HYPERSONICS in some encyclopedia (Britannica?) years ago. It connected up two separate fields of physics for me. It seems that as the frequency of sound goes up, at some very high point the wavelength becomes nearly as short as the spacing between atoms. In this range of frequencies the speed of sound drops radically, until the sound "oozes" rather than propagating as a wave, and moves as incoherent, interacting vibrations. Also, the wavelength of the sound is similar to the wavelength of longwave IR radiation. The upshot: "heat energy" is the regime where sound has become so high in frequency that it moves slowly and is renamed as "thermal vibrations." And the wavelength of the sound is so short that it can interact with individual atoms and be converted to IR light. My concept of heat energy within solids has been changed. Now I imagine it to be a very very loud "hiss" of white noise, high frequency sound which propagates slowly through objects, and which also induces the objects to glow with invisible light. We live in a strange world, no?

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