Question

Why we use sodium light in newton ring experiment instead of using white light?

Answer 1

Answer:

Because the optical path (the difference in path length between the upper and lower reflected rays) needs to be an integral number of wavelengths for constructive interference and an odd number of half wavelengths for destructive interference.

White light contains many different wavelengths so one doesn't get well defined fringes.

Sodium light consists of a single wavelength of  5892 A where 1 Angstrom equals 1 * 10E-10 m. So the sodium light can produce well defined fringes.

(Actually, the sodium light consists of 2 wavelengths, but they are very close together and suitable for the Newton's rings experiment.)