Your job is to design a thin film coating that will perform a given function. The coating has an index of refraction of 1.36 and it is deposited on a glass substrate that has an index of refraction of 1.48
Question #1:
Which of the two rays shown below will have a one half wave phase shift upon reflection-
a. the ray reflected from the top onlyb. the ray reflected from the bottom of the film only
c. neither ray
d. both rays sinc both are reflected by a HIGHER index of refraction
Question #3 Be sure to explain your reasoning!
PART A How thick should the film be so that it strongly reflects light of 400 nm? That is, when you look at the film head on it will look violet.
In the film, the wavelength of light is 400 nm/1.36= 294 nm.
Since BOTH rays have a change of phase on reflection, we can ignore this effect. That is, if the film were very, very thin (say 1 nm) so that there was essentially no difference in the path lengths, both rays would emerge in phase.
What is the next thickness that allows them to emerge in phase? The wave that goes farther must go one whole wavelength farther. That is, the film must be one half wavelength thick- (1/2) (294 nm) = 147 nm.
PART B How thick should the film be so that it does NOT reflect light of 400 nm? That is, when you look at the film head on, all colors but violet will reflect (400 nm "anti reflects")
In the film, the wavelength of light is 400 nm/1.36= 294 nm.
Since BOTH rays have a change of phase on reflection, we can ignore this effect. That is, if the film were very, very thin (say 1 nm) so that there was essentially no difference in the path lengths, both rays would emerge in phase.
When is the next thickness that allows them to emerge in phase? The wave that goes farther must go one whole wavelength farther. That is, the film must be one half wavelength thick- (1/2) (294 nm) = 147 nm.