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. `2.945 xx 10^(-7)`
B. `3.945 xx 10^(-7)`
C. `4.95 xx 10^(-7)`
D. `1.945 xx 10^(-7)`

Answer

A

Explanation

Thin film appears dark,
`2mut= nlambda implies t= (nlambda)/(2mu)`
`implies t_("min")= (lambda)/(2mu)= (5890 xx 10^(-10))/(2 xx 1)= 2.945 xx 10^(-7) m`