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Physics The Nature of Light Experiment: Thin Film Interference Patterns Background Information A soap bubble seems to have an ever-changing pattern of colors flowing across its surface. Thin oil films, especially on water, will display the same colors. The colors are caused by interference of light, not by some pigment in the oil or soap. The interference occurs because light reflects off the soap or oil as it enters the film, and then again when it leaves the film. Interference can be either constructive or destructive. Constructive interference occurs when two waves meet crest to crest or trough to trough, creating a larger amplitude wave. These waves are said to be “in phase” because they are doing the same thing at the same time. Destructive interference occurs when two waves meet crest to trough and thereby cancel each other out. These waves are said to be “out of phase” because they are doing opposite things. Two reflections occur as light goes into a film and out on the other side. Two reflected waves are present, so interference will occur. The phase of the reflected wave is sometimes changed and sometimes remains the same. When light goes from a “fast” medium to a “slow” medium, the reflection from the surface will be 180° out of phase. When the opposite occurs, the reflection is in phase. Therefore, as a ray of light hits the outside of the soap bubble, it is reflected out of phase and when it leaves the film and enters the air inside the bubble it reflects in phase.



Bubble Outer Surface

Bubble Inner Surfa


In the diagram above, Ray 1 reflects off the outside surface to your eye and ray 2 reflects off the inside surface to your eye. The path of ray 2 is longer than the path of ray 1. Also, one reflection is in phase while the other is 180° out of phase. If ray 2 traveled an extra distance of ½ wavelength, it would then change its phase by another 180°. This will be a total of 360° so the waves will be in phase. Constructive interference will cause that wavelength to be visible. If the light travels an extra distance of one wavelength, for example, the waves would be out of phase and no color would be visible. If the thickness of a bubble or an oil film is uniform, only one wavelength will come back to your eye in phase. Therefore, only one color will be seen. Different colors are seen in different places because the soap film has a difference thickness in different places. The colors swirl and flow on the bubble because the water is flowing down the bubble under the influence of gravity and the bubble gets thinner at the top and thicker at the bottom. Oil films show colors in the same way. They are made of a thin film and light travels more slowly in oil than in water or

Experiment: Thin Film Interference Patterns, Procedure, page 2 air. The reflection at the air-oil surface is out of phase and the reflection at the oil-water surface is in phase.

Purpose The purpose of this investigation is to observe and develop a hypothesis about the phenomenon of thin-film interference patterns. Equipment container of bubble solution* tape plastic straw 1 large black plastic serving tray 3 sheets of white notebook paper *Note: To make bubble solution, mix 1/2 cup Ultra Dawn brand dishwashing detergent and 1 tablespoon glycerin in 1/2 gallon of water. It’s best to let this solution stand overnight. Procedure 1.

Place the plastic serving tray on the work area.


Prepare a “white collar” by taping three sheets of white paper together so they form a cylinder 8-1/2 inches high. The white collar reduces air currents and reflects light onto the bubble so its color can be clearly seen.


Wet the serving tray with the bubble solution.

4. Place the white collar on top of the soapy area. 5. Dip a straw into the solution on the work area. 6. Gently blow through straw to form a bubble dome. 7. Remove the straw from the bubble. 8. Observe the changing colors you see on the top of the bubble. 9. Continue to blow bubbles as you answer the questions on your laboratory report.

Physics The Nature of Light Experiment: Thin Film Interference Patterns Data/Observations 1. Below list the color sequence of five (5) different bubbles: Bubble 1: Bubble 2: Bubble 3: Bubble 4: Bubble 5: 2. Below note observations concerning the apparent “swirling” of the colors on the soap film.

3. What observations do you make just before the bubble “pops?”

Questions 1. What is the purpose of the “white collar?”

2. List, in order of appearance, the following colors: black, blue, green, magenta, white and yellow.

Experiment: Thin Film Interference Patterns, Student Handout, page 2 The figure below represents a thin film of uneven thickness, such as a soap bubble, with a beam of light approaching. The film reflects some of the light and transmits some of the light as shown by the ray arrows. 1


Bubble Outer Surface

Bubble Inner


3. Ray 1 has been A. refracted only. B. reflected only. C. reflected and refracted. 4. Ray 2 has been A. refracted only. B. reflected only. C. reflected and refracted. 5. The two emerging rays A. must reinforce each other. B. must cancel each other. C. may reinforce each other. 6. In order for light to interfere, more than one wave is necessary. In the case of bubbles, light is reflected from both the ___________________ surface and the ______________ surface. 7. What causes the “swirling� of the colors on the soap film?

8. The thickness of a film determines which color is visible. Explain.

9. Where do the colors in the bubble come from?

10. Why does the bubble appear white with growing black spots just before it pops?