(c)1996 William J. Beaty

Explanation and demonstrations.

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, magenta, etc.)

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.

Another trick:
(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!
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