Question

Two plano convex lens each of radius 100 are placed with their curved surfaces in contact with each other. Newton's rings are formed by a light of wavelength 6000A°. Find the distance between 10th and 20th fringe.

Answer 1

We can write for the radius of dark ring

$r_{k}=\sqrt{kRλ} (1)$

In our two cases

$r_{10}=\sqrt{10Rλ} (2)$

$r_{20}=\sqrt{20Rλ} (3)$

The distance between 10th and 20th fringe is equal to

$Δr=\sqrt{20Rλ}-\sqrt{10Rλ} (4)$

Using (4) we get

Δr=8.83×10^-3 m

Answer

8.83×10^-3 m