Thin films: Light of wavelength 425.0 nm in air falls at normal incidence on an oil film that is 850.0 nm thick. The oil is floating on a water layer 15 cm thick. The refractive index of water is 1.33, and that of the oil is 1.40. You want to add oil so that light reflected off of the top of the oil film will be canceled. What is the minimum distance that you should INCREASE the oil film?

Question

Quizplus · Accepted Answer

The correct answer is A.The light reflected off of the top of the oil film will be canceled if the path length difference between the light reflected off of the top of the oil film and the light reflected off of the bottom of the oil film is equal to half a wavelength. The path length difference is equal to 2nt, where n is the refractive index of the oil and t is the thickness of the oil film. Therefore, the minimum distance that you should increase the oil film is equal to lambda/4n, which is 60.7 nm.

Thin Films: Light of Wavelength 425