Homework #10: THIN FILMS and INTERFEROMETERS

1. Give a practical application for each of the following:

a. Michelson interferometer

b. Mach Zender interferometer

c. Fabry Perot interferometer

d. thin film coating

e. fiber Bragg grating

2. A film of oil is smeared on a glass plate. A source of monochromatic light illuminates the film, and the observer looking straight down at the top of the film sees the two waves as they recombine after reflecting from the top and bottom surfaces of the film.

Answer the following questions based on the figure below:

a. At which surface(s) does the light wave undergo a change of phase?

b. If the film is very, very thin (the distances traveled by the two waves are approximately the same) will the waves be in phase or out of phase when they reach the observer?

c. Suppose the light has a wavelength in a vacuum of 600 nm. What is the wavelength of the light in the film?

d. If the film is 100 nm thick, how many waves farther does the light that reflects from the bottom travel compared to the light reflected from the top? (i.e., what is the path length difference in wavelengths?) Be sure to use the wavelength in the film.

e. Does the 600 nm light reflect strongly (the observer sees it) or not at all (it cancels).

3. EXTRA CHALLENGE (optional). Would the answer be different if the oil film were floating on water? Why?