A light of wavelength `5890Å` falls normally on a thin air film. The minimum thickness of the film such that the film appears dark in reflected light is

Options

A. (a) `2.945xx10^-7m`
B. (b) `3.945xx10^-7m`
C. (c) `4.95xx10^-7m`
D. (d) `1.945xx10^-7m`

Answer

A

Explanation

If thin film appears dark
`2mutcosr=nlambda` for normal incidence `r=0^@`
`implies2mut=nlambdaimpliest=(nlambda)/(2mu)`
`impliest_(min)=(lambda)/(2mu)=(5890xx10^(-10))/(2xx1)`
`=2.945xx10^-7m`.