Question

A light ray is incident normal to a thin layer of glass. Given the figure, what is the minimum thickness of the glass that gives the reflected light an orangish ***** (λ(air) orange light = 600 nm)

A) 50 nm

B) 100 nm

C) 150 nm

D) 200 nm

E) 500 nm

Answer 1

The correct option is: (B) 100 nm

Explanation:

First find the λ in the film. nair λair = nfilm λfilm … (1)(600) = (1.5) λglass … λglass = 400 nm As the light travels through the two boundaries, you get a λ phase shift (flip) at the first boundary but no shift at the second boundary. Therefore, you need to make another λ of phase difference total by traveling in the film thickness to produce constructive interference to reinforce the orange wavelength. When the glass thickness is of the λ in the glass, the light will travel up and down to make the extra λ needed. So of the λ in the glass gives you 100 nm thickness needed.