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.
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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