Visible light of variable wavelength is incident normally on a thin sheet of plastic in air. The reflected light has a minima only for `lamba=512` nm and `lambda=640` nm in the visible spectrem. What is the minimum thickness (in mum) of the film `(mu=1.28)` ?

Answer

`0001`

Explanation

Reflected ray from upper surface would would shift by `lambda//2` only while reflected from lower surface would not have any shift.
`2mut=n_(1)lambda_(1)=n_(2) lambda_(2) Rightarrow (n_(1)=n_(2)+1)`as there is no minima in between these two wavelengths
`(n+1)(512)=(n)(640)`
`n_(2)(640-512)=512`
`n_(2)=4`
So `2xx1.28t=(4)(640)`
`t=(4xx640)/(2xx1.28)=1000` nm=1mum