Information in a compact disc is stored in pits whose depth is essentially one-fourth the wavelength of

Question:

Information in a compact disc is stored in “pits” whose depth is essentially one-fourth the wavelength of the laser light used to “read” the information. That wavelength is 780 nm in air, but the wavelength on which the pit depth is based is measured in the n = 1.55 plastic that makes up most of the disc. Find the pit depth