A glass lens, n glass = 1.52 , has a 131 nm thick antireflective film coating one side, n film = 1.25. White light, moving through the air, is perpendicularly incident on the coated side of the lens. What is the largest wavelength of the reflected light that is totally removed by the coating?

655 nm

Explanation:

When the intereference is destructive then the thickness, d of antireflective film coating one side is given by

2d=w/2n

Where w is wavelength and n is the reflective index of the film

Making w the subject of formula then

w=4nd

Substituting 1.25 for n and 131 nm for d then the wavelength will be

w=4*1.25*131=655 nm

Therefore, the wavelength is equivalent to 655 nm

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