Question

A plano-convex lens of radius $3m$ is placed on an optically flat glass plate and is illuminated by monochromotic light. The radius of the $8^{th}$ dark ring is $3.6mm$. Calculate the wavelength of light used.

Answer 1

$r_n^2=nR\lambda \left(or\right)\lambda =\frac{r_8^2}{8R}$
$\lambda =\frac{3.6\times {10}^{-3}\times 3.6\times {10}^{-3}}{8\times 3}$
$=5400\mathring{A}$ (OR) $5.4\times {10}^{-7}m$