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# Colors and Thin-Film Interference (page 2)

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Author: Janice VanCleave

### Try New Approaches

1. How does the thickness of the film affect the results? Repeat the experiment using a small container of water, such as a bottle cap. For information about interference in thick films, see Craig F. Bohren, What Light through Yonder Window Breaks? (New York: Wiley, 1991), pp.18–19.
2. Does the material that the thin film is made of affect the results? Repeat the experiment using different oils, such as cooking oil, baby oil, and motor oil.
3. How does motion of the material affect the results? Use a toothpick to gently move the material around on the surface of the water as you observe the light reflecting from it.

1.
1. A soap bubble is a combination of soap and water linked together to form a thin layer of elastic liquid surrounding air. Investigate interference colors in the film of soap bubbles. Prepare a bubble solution by combining 1/4 cup (63 ml of dishwashing liquid and 1 cup (250 ml) of tap water in a small bowl. Gently stir. Dip one end of a drinking straw into the bubble solution and blow through the straw. Caution: Take care to blowout and not suck in the soapy liquid. Set the bowl of bubbles near an incandescent light and observe the appearance of the bubbles.
2. Does the angle of the light striking the film affect the results? Design a way to control the angle of the incident light, such as placing the bowl in a box with one hole for light to enter and another hole for viewing.
2.
1. What effect does the thickness of a soap bubble have on the color produced by interference? Design a way to change the thickness of the soap film. One way is to use gravity. Dip a wire loop into the bubble solution. Holding the loop vertically in front of a dark background, such as a sheet of black paper, watch for colors to appear in the film. Note the difference in colors and patterns at the top and the bottom of the film in the loop. Also, note the color of the film just before it breaks.
2. Another way to measure the angle of the light is to change the angle of the wire loop in relation to a stationary light. Design a way to measure this angle. Photographs of the film could be used to represent the procedure and the results.
3. Does the light source affect interference? Repeat the experiment using different light sources, such as fluorescent light and sunlight. For information about the effect of light sources, see Craig F. Bohren, What Light through Yonder Window Breaks? (New York: Wiley, 1991), pp. 21-22.

### Get the Facts

1. Constructive and destructive interference depend on the thickness of the film. What is destructive interference? What thickness satisfies the requirement for constructive and destructive interference of a specific wavelength? For information on the thickness that produces constructive interference, see Robert L. Lehrman, Physics the Easy Way (Hauppauge, N.Y.: Barron's, 1998), pp. 413-414.
2. British scientist Sir Isaac Newton (1642-1727) proposed the corpuscular theory of light. Later, another British scientist, Thomas Young (1773-1829), proposed the wave theory to explain light. How do these two theories differ? Which is used today? Which is used to explain the colors in a thin film? For information, see Tony Rothman, Instant Physics (New York: Fawcett Columbine, 1995), pp. 102-107.
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