Question

A thin oil film of refracting index `1.2` floats on the surface of water `(mu=4/3)`. When a light of wavelength `lamda=9.6xx10^(-7)m` falls normally on the film air, then it appears dark when seen normally. The minimum change in its thickness for which it will appear bright in normally reflected light by the same light is `Zxx10^(-7)m`. Then find `Z`.
A. `10^(-7)`m
B. `2xx10^(-7) m`
C. `3xx10^(-7)m`
D. `5xx10^(-7)m`

Answer 1

Correct Answer - B
For normal incidence path difference between ray 1 and ray 2 is `2 mu_(1)t`
For minima thickness increment `2 mu_(1) Deltat=(lambda)/2`
`rArr (t_(2)-t_(1)) =(lambda)/(4mu_(1)) =(9.6xx10^(-7))/(4xx1.2) =2 xx10^(-7) m`