Question

The ratio of the radii of the 4th and 9th dark ring in newton's ring experiment is

Answer 1

The condition for constructive interference in thin film is,

m = 0,1,2,....

From equation (1) putting the value of t in the above equation we get,

since n = 1 for air film

or

For first bright ring, m = 0

For second bright ring, m = 1

For third bright ring, m = 2

Similarly, for Nth bright ring,

m = N - 1

Radius for dark ring

The condition for destructive interference in thin film is,

2tn = mλ m = 0,1,2,....

By putting the value of t, we get,

For air n = 1

For i.e. point of contact.

Now, if the radius of curvature of plano-convex lens is known and radius of particular dark and bright ring is experimentally measured then the wavelength of light used can be calculated from equation (3) and (4).