A Surface Is Coated with a Material Having Index of Refraction

Multiple Choice

A surface is coated with a material having index of refraction 1.50.If light in air has a wavelength of 450 nm and is normally incident on this surface,and it is found through interference effects with this light that the surface is 15 wavelengths thick,which of the following is the thickness of the surface?

A) 6.8 μm
B) 1.5 μm
C) 3.0 μm
D) 4.5 μm