Question

Part A A thin film of soap with n = 1.38 hanging in the air reflects dominantly red light with 2 = 648 nm. What is the minimum thickness of the film? Submit Answer Tries 0/12 Part B Now this film is on a sheet of glass, with n = 1.55. What is the wavelength of the light in air that will now be predominantly reflected?

Answer 1

Explanation:-

Making a couple of assumptions you are in are in a position to see the light reflected from both the front and back surfaces of the film and that the two rays constructively interfere for this wavelength for the hanging film the light reflected from the front surface is phase inverted because it is going from low n to higher n and the ray from the back is not inverted (nigh n to lower n) so the path difference needs to be 1/2 wavelength to put them back in phase (constructive interference)

that means the minimum thickness must be 1/4 wavelength 1/4 * 648= 162nm

when backed by glass the ray from the rear surface phase shifted

(lower n to higher n)

so the total path length must be 1

wavelength to put the two rays back in phase

so the thickness must be 1/2 wavelength

1/2 * 648= 324 nm