setup with double thin slits is illuminated by a source of coherent, plane light waves

such as a laser, just as was done during several lecture demonstrations.

Later, a thin, $100\%$ transparent film is put in front of one of the slits. The thickness of the film is $0/($n$-1),$

where $0$ is the wavelength of the incident light and n is the index of refraction of the film.

a$)(3$ pts$)$ Describe in one or two coherent sentences the interference pattern observed on a screen far

away before the film is put in front, in as much detail as possible given the limited information.

b$)(5$ pts$)$ After the film is put in front, what is the difference in phase Df between the light reaching the

two slits?

c$)(2$ pts$)$ Describe what happens to the interference pattern seen on the screen as a result of putting the

film in front, and explain why.