Question

Interference Colors

(a) Why is reflection from a soap film much like reflection from an oil layer? How is it different?

(b) With reference to the interference color in reflected light, a very thin soap film is sometimes called a “black film”. Why would it have a name like that? Is there such a thing as a “white film”?

Answer 1

a] Both Soap film and an oil layer can be categorized as thin films which essentially introduces a path difference between reflected light from upper and lower interfaces. This causes interference and the thickness of the film decides for a given wavelength if the light will constructively or destructively interfere.

They yet different in terms of their refractive indices. The path length of light is dependent on the refractive index of the material it passes through and so increasing the refractive index of a material with same thickness amounts to effectively increase the path length thus changing the interference order.

b]

For constructive interference,

=>

so as the thickness t of the film decreases, the wavelength getting reflected constructively decreases till the point where the wavelength is smaller than the wavelength of the visible region. Now, no light in visible region gets reflected [since any higher order m would make the wavelength even smaller] and so the film appears black.

A white film can only exist if the thickness of the film is many order greater than the wavelengths in the visible region. For the thickness of the soap films, a white film would imply that all the wavelengths are getting constructively reflected which cannot happen in interference.