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Colors and Thin-Film Interference

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Author: Janice VanCleave

Written references to colors being seen in thin films of materials, such as oil on water, have been found on clay tablets dating back several thousand years. Some ancient cultures used the colored patterns produced by oil on water for fortune-telling, but scientists, including Sir Isaac Newton, devised experiments to explain the production of the colors.

In this project, you will observe colors produced by thin films and discover what effect different materials used for the film and the motion of the film have on the colors produced. You will also investigate interference colors in a film of soap bubbles and how the colors are affected by the thickness of the film, the angle of the incident light, and the type of light source.

Getting Started

Purpose: To observe colors in a thin film.

Materials

• opaque cereal bowl (opaque-not capable of being seen through)
• tap water
• desk lamp with incandescent bulb
• colorless nail polish

Procedure

1. Fill the bowl about three-fourths full with water.
2. Set the bowl near but not directly under a desk lamp.
3. Let one drop of colorless nail polish fall on the water's surface in the bowl.
4. Turn the desk lamp so that its bulb is about 6 inches (15 cm) above the table and is pointing down.
5. Stand so that you can see the surface of the water in the bowl from above as you slowly move the bowl toward the lamp until the bowl is beneath it. Make note of any color changes in the nail polish on the. water's surface.

Results

Thin bands of colors are seen in the nail polish in the bowl.

Why?

The drop of nail polish spreads out, forming a very thin transparent (so clear that it allows light to pass straight through) film on the water's surface in the bowl. White light, such as that from the incandescent bulb of the desk lamp, contains all the visible colored light waves. When white light hits the thin, transparent film, some of the light reflected from the outer surface of the film (wave 1) and some of it entered the film and was reflected from the inner surface (wave 2) (see Figure 26.2). Even though the difference in distance was small, the light reflected from the inner surface traveled a slightly longer distance than the light reflected from the upper surface.

Both reflected waves 1 and 2 are white light with less intensity than the incident light. When the two reflected waves come back together, superposition (placing one thing on top of the other) occurs. The separate reflected waves undergo superposition if they are focused by a lens. This occurs in the eye when the eye's lens focuses the light onto the retina. The superposition of one wave on another is called interference. Constructive interference occurs for wavelengths in the reflected waves that are in phase (in step). Constructive interference is the superposition of two or more waves in phase, resulting in a combined wave with an amplitude larger than the component waves (see Figure 26.3). For example, if constructive interference occurs for the waves of red light in the two reflected waves, the part of the film reflecting these waves has a red color because the red light wave in this area has a much great amplitude and thus is more intense than the other colors of light. Reflected light waves from other parts of the film might have constructive interference for the green light waves; thus this region of the film would appear green, and so on for regions of the film and other colors in the white incident light.

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