Monochromatic Light, at Normal Incidence, Strikes a Thin Film in Air.If

Multiple Choice

Monochromatic light, at normal incidence, strikes a thin film in air.If $\lambda$ denotes the wavelength in the film, what is the thinnest film in which the reflected light will be a maximum?

A) "much less than $\lambda$ "
B) " $\lambda$ /4"
C) " $\lambda$ /2"
D) "3 $\lambda$ /4"
E) " $\lambda$ "