Question

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

Answer 1

Given data:

$R=3m;n=8$;

$r_8=3.6$mm$=3.6\times {10}^{-3}$; $\lambda =?$

Solution:

$r_n=\sqrt{nR\lambda }$

$\Rightarrow r_n^2=nR\lambda$

$\lambda =\frac{r_n^2}{nR}=\frac{(3.6\times {10}^{-3}{)}^2}{8\times 3}$

$\lambda =5400\times {10}^{-10}m$

or

$\lambda =5400\stackrel{o}{A}$

.