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

Answer

A

Explanation

If thin film appears dark
`2 mu t cs r = n lambda` [for normal incidence `r = 0^(@)`]
`2 mu t = n lambda rArr t = (n lambda)/(2 mu)`
`t_("min") = (5890 xx 10^(-10))/(2 xx 1) = 2.945 xx 10^(-7)m`