Question

Please explain and write out clearly!

Soap bubbles appear to be a mixture of red and green colors when sunlight strikes the wall of a soap bubble at nearly normal incidence (red 700 nm, green 500 nm).
a) Explain how the soap bubble can look both red and green at the same time. In your explanation, answer whether this is due to constructive or destructive interference.
b) What is the thickness of the bubble wall if the inex of refraction of the bubble is 1.35?

Answer 1

White light is made up of all colors, all wavelengths. The skin of a bubble glistens with the complementary colors produced by interference. If we were to look at a highly magnified portion of a soap bubble membrane, we would notice that light reflects off both the front (outside) and rear (inside) surfaces of the bubble, but the ray of light that reflects off the inside surface travels a longer distance than the ray which reflects from the outside surface. When the rays recombine they can get "out of step" with each other and interfere. Given a certain thickness of the bubble wall, a certain wavelength will be cancelled and its complementary color will be seen. Long wavelengths (red) need a thicker bubble wall to get out of step than short wavelengths (violet). When red is cancelled, it leaves a blue-green reflection. As the bubble thins, yellow is cancelled out, leaving blue; then green is cancelled, leaving magenta; and finally blue is cancelled, leaving yellow. Eventually the bubble becomes so thin that cancellation occurs for all wavelengths and the bubble appears black against a black background.

The iridescence of a soap bubble, which seems to contain a wealth of changing color, stems from light striking the bubble from varied angles. The path length varies with the angle of incident light, giving varying path differences for the internally and externally reflected rays at different points on the bubble. This means that, even if the soap film is of uniform thickness, different colors can be seen. Light entering the bubble directly travels a shorter path than light entering at a wider angle. This allows different wavelengths to undergo constructive and destructive interference, so different colors are perceived.