Red light having a wavelength of 690 nm (in air) is vertically incident on a thin film of oil that has an index of refraction of 1.19. The film of oil floats above water (having an index of refraction of 1.30). The red light is brightly reflected. The minimum (nonzero) thickness of the film of oil is
A) 290 nm.
B) 345 nm.
C) 580 nm.
D) 145 nm.

Question

Quizplus · Accepted Answer

The minimum thickness of the film of oil is one-quarter of the wavelength of the light in the oil. The wavelength of the light in the oil is 690 nm / 1.19 = 580 nm. Therefore, the minimum thickness of the film of oil is 580 nm / 4 = 145 nm.

Red Light Having a Wavelength of 690 Nm (In Air)