A thin oil film of refractive index 1.2 floats on the surface of water `(mu = 4/3)`. When a light of wavelength `lambda = 9.6xx 10^(-7)m` falls normally on the film from 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:

Answer

2

Explanation

For normal incidence path difference between ray `1` and ray `2` is `2mu_(1)t`
For minimum thickness increment `2mu_(I)Deltat=(lamda)/2`
`implies(t_(2)-t_(1))=(lamda)/(5mu_(1))=(9.6xx10^(-7))/(4xx1.2)=2xx10^(-7)m`