Tuesday, March 07, 2006

Thin Film Interference

Experiment - Making a Permanent Film

In this experiment we will make a thin film on a piece of cardboard that is far less transitory than oil on water. Then you will be able to investigate the properties of the film interference, like the color and angle dependence. Also, just in case you didn't believe me about the thin film and think that maybe the rainbows come from the colors in the colored oil separating somehow, we're going to use something that is definitely clear - nail polish. And, as we'll be allowing it to dry and harden, there is no chance that the changing colors can be caused by movement of different colored parts of the oil, say.

Things That You Will Need

    A bowl of water.

    Thin cardboard


    Clear nail polish

    String or fishing line

    Safety pin, hole punch or stapler.


    1) Cut a shape from the cardboard. It can be any shape you like, but try to make it only a few centimetres across.

    2) Punch a hole in the card and tie the string securely to the shape through the hole.

    3) Place the shape into the bowl, so it is completely submerged, with the string hanging out of the bowl. If it doesn't completely sink straight away, you should hold it under briefly while it absorbs enough water to weigh it down.

    4) Place a drop of nail-polish onto the surface of the water. It will probably stay as a drop for a few seconds, then suddenly spread out to make a puddle on the surface of the water, as its surface tension lets go.

    5) As soon as the nail polish has spread out, pull the shape straight up out of the water, through the nail polish. If it ends up with a slimy-looking trail hanging off it, cut this off after it has dried.

    6) Hang the shape up to dry. As it's drying, look at the light reflecting off the surface. You should be able to see the thin film interference from the layer of wet nail polish.

    7) As the polish dies, it will keep its thin film characteristic and remain locked in this shape forever. The irregularities in the film will lead to a swirly effect much like for oil on water - perhaps you can think of ways this proceedure can be modified to minimize the irregularity.

When it has dried there are several things you can think about:

    1) hold it so you can see light reflecting off its surface. Try holding it at different angles and see how the colors change. As you tilt the sheet away from you, the path difference will increase. This means that the perfect wavelength for a particular part of the film will also get longer (
    untill the path difference once more corresponds to a short part of the visible s
    pectrum). From the order of colors you see as you tilt the card, what can you infer about the wavelengths of the colors?

    2) As each part of the film reflects only a specific color (at a fixed angle), you have made a crude interference filter - in essence a devide that tells you how much of each color light is in the light that strikes it. For example, in sun light or under reasonable white light illumination, you should get all the colors (with a perfect film - it is quite posisble that irregularities might make it hard to observe some colors with your particular film), where as when you look at the yellow sodium street lights, you might be able to see the narrow bands of yellow (and a few other colors) that distinguish sodium fom other elements.

    3) What other experiments can you think of to utilize your film?

Oil on Water

When oil (or soap) sits on top of water, it takes on a rainbow like appearance. Colors swirl and mix as the surface of the water ripples. Do these colors come from the oil directly? No, in fact, the colors you see are not really "the colors of oil" in the normal sense of the word - if you look at oil in its usual form, it tends to only have one color. Depending on the type of oil, it might be dark or light brown, yellow or perhaps a solid bright unnatural color, like blue (bright colors are dyes added by the manufacturer). It is only when the oil lies on top of water or some smooth surface like cement or glass that these colors can be seen.

Why is that?

If you have ever poured oil onto water, you will have seen another important property of oil - it floats. Most

oils are lighter (or more precisely, less dense) than water, and thus float on top of it. So, when oil is poured onto water, it floats on top. But, because gravity pulls the oil down and its buoyancy in water pushes it back up, it spreads out untill it forms a film over the water that is very, very thin.

These films tend to be only a few wavelengths of light thick - about 1 micrometer (one millionth of a meter) thick. When light hits the surface of the oil, some of it is reflected (the reflected wave) and some of it goes through the surface (the refracted wave), bounces back off the water and re-emerges out the top of the oil (see figure).

How the Film Determines its Colors

When the light bouncing off the surface of the oil combines with the light coming out of the surface, they w

ill either cancel each other out or reinforce each other (make each other stronger). This is called interference. Whether they cancel out or reinforce one another depends on two things: the color of the light and the distance the light travels inside the oil.

Constructive Interference

The color deter mines the wavelength, and if the extra distance traveled by the refracted beam relative to the reflected beam - the path difference is a whole number of wavelengths, then the peaks and troughs of both waves align exactly, or are "in phase", and the waves reinforce each other (see figure). This is called constructive interference.

Destructive Interference

At the other extreme, if the extra distance traveled is exactly between two whole multiples of the wavelength, the peaks of the reflected wave will align with the troughs of the refracted wave (or are "180o out of phase") and they will exactly cancel out so none of this light comes through (see figure). This is called destructive interference. For distances in between these two extremes, the waves partially cancel and partially reinforce, depending on how far "out of phase" they are.

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