q a plano-convex lens of radius 3 m is placed on

Question

Q. A Plano-convex lens of radius 3 m is placed on a flat glass plate and is illuminated by monochromatic light. The radius of the 8th dark ring is 3.6 mm. Calculate the wavelength of the light used.

Data : R = 3m ; n = 8 ; r8 = 3.6 mm = 3.6 × 10-3 m ; λ = ?

Solution : r_n = √ nRλ

r_n²= nRλ

λ = r_n² / n R = (3.6 X 10^-3 )²/ 8 X 3

= 5400 × 10^-10 m (or) 5400 A⁰