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

Multiple Choice

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.