Question

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

Answer 1

Correct Answer - D
For reflected light
`2 mut cos r-lambda/2=(2n+1)lambda/2`
`2 mu t cos r=(2n+1)lambda/2+lambda/2, n=0`
`2xx1.5xxt cos 60^(@)=lambda=6000`
`t=6000/1.5=4000 Å`