Athin film of oil (n = 1.27) is located on smooth, wet pavement. when viewed perpendicular to the pavement, the film reflects most strongly red light at 640 nm and reflects no blue light at 427 nm. how thick is the oil film?

You are given a thin film of oil (n = 1.27) that is located on smooth, wet pavement. Also, it is said that it is viewed perpendicular to the pavement and that the film reflects most strongly red light at 640 nm and reflects no blue light at 427 nm. You are asked to find the thickness of the oil film.

The first thing you need to do is to divide the wavelength of the red light by the index of refraction of the oil to determine the wavelength of the red light in the oil. Then you will use 2t = mλ where t is the thickness, λ is the wavelength and m is the magnification. Note that it is viewed in a perpendicular pavement, m = 1 because it is a constructive interference. 

2t = mλ
2t = (1) (640nm/1.27)
2t = 503.94 nm
t = 251.97 nm