A parallel beam of sodium light of wavelength `6000 Å` is incident on a thin glass plate of `mu=1.5`, such that the angle of refraction in the plate is `60^(@)`. The smallest thickness of the plate which will make it appear dark by reflected light is

Options

A. `4000 Å`
B. `40 Å`
C. `400Å`
D. `4 Å`

Answer

A

Explanation

The condition for minimum thickness corresponding to a dark band is, `2mu t" cos"r=lambda`
` :. t=(lambda)/(2mu"cos"r)=(6000xx10^(-10))/(2xx1.5xx"cos"60^(@))`
` " "=4000xx10^(-10)m=4000Ã…`