Question

A glass of refractive index $1.5$ is coated with a thin layer of thickness of $t$ of refractive index $1.8$ light of wavelength $\lambda$ tavelling at the upper and the lower surfaes of the layer and the two reflected rays interfere. If $\lambda =648nm$, obtain the least value of $t$ (in ${10}^{-8}m$) for which the rays interfere constructively.

Answer 1

For constructive Interference:

$2\times \mu \times t=(2n+1)\frac{\lambda }{2}$

$2\times 1.8\times t=\frac{640\times {10}^{-9}}{2}$

$t=9\times {10}^{-8}m$