Rainbows in Soap Bubbles
Have you ever wondered why soap bubbles are rainbow colored, or why an oil spill on a wet road has rainbow colors in it? This is what happens when light waves pass through an object with two reflective surfaces. When two incoming light waves of the same frequency strike a thin film of soap, as seen in Figure 5 below, parts of the light waves are reflected from the top surface, while other parts of the light pass through the film and are reflected from the bottom surface. Because the parts of the waves that penetrate the film interact with the film longer, they get knocked out of sync with the parts of the waves reflected by the top surface. Physicists refer to this state as being out of phase. When the two sets of waves strike the photoreceptors in your eyes, they interfere with each other; interference occurs when waves add together or subtract from each other and so form a new wave of a different frequency, or color.

Basically, when white light, which is a mixture of different colors, shines on a film with two reflective surfaces, the various reflected waves interfere with each other to form rainbow fringes. The fringes change colors when you change the angle at which you look at the film, because you are changing the path by which the light must travel to reach your eye. If you decrease the angle at which you look at the film, you increase the amount of film the light must travel through for you to see it. This causes greater interference.

Figure 5

Everything we see is a product of, and is affected by, the nature of light. Light is a form of energy that travels in waves. Our eyes are attuned only to those wave frequencies that we call visible light. Intricacies in the wave nature of light explain the origin of color, how light travels, and what happens to light when it encounters different kinds of materials.


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  • Serway, Raymond A, and Jerry S. Faughn, (1999) Holt Physics, Holt, Rinehart, and Winston, Austin, Texas