INTERFERENCE BY THIN FILMS Young was able to explain the colors arising from thin films: the colors of soap bubbles and oil slicks on a wet pavement, the rainbow that appears on oxidized metal surfaces, etc.

The dynamic interplay of colors is generated by the simultaneous reflection of light from both the inside and outside surfaces of the bubble. The two surfaces are very close together, but the light reflected from the inner surface of the bubble must still travel further than light reflected from the outer surface. When the waves reflected from the inner and outer surface combine they interfere with each other, removing or reinforcing some parts of white light, resulting in the appearance of color.

If the extra distance traveled by the inner light waves is exactly the wavelength of the outer light waves, constructive interference occurs and bright colors of those wavelengths are produced. In places where the waves are out of step, destructive interference transpires, canceling the reflected light and the color.

Thin transparent films Thin transparent films generate fringe patterns when the two waves reflected from the front and back surfaces interfere. Where the film is non-uniform, there is a pattern of fringes of equal thickness. CASE 1. When the index of refraction of the film lies between the indices of the surrounding two media: n 1 > n f >n 2 or n 1 < n f <n 2 reflected maxima occur where the thickness (d) is: m = 1, 2, 3,…

An even more commonly occurring situation is for: CASE 2. When the index of refraction of the film is greater or less than the indices of the surrounding two media: n 1 n 2 or n 1 > n f <n 2 Now maxima occur where: m = 0, 1, 2,…

10.11 A soap film in air has an index of refraction of If a region of the film appears bright red (λ o =633 nm) in normally reflected light, what is its minimum thickness there? n f = 1.34 λ = 6.33x10 -7 m m = 0 This is an example of Case 2. 1 = 1.18x10 -7 m