Question

Why the central fringe is dark in Newton's ring?

Answer 1

Interference:

1. Interference is a phenomenon in which two waves combine to form a resultant wave so that their displacement at every single point and time is added.
2. When two waves with the same phase overlap, it is called constructive interference. They produce bright spots.
3. When two waves out of phase overlap each other, it is called destructive interference. They produce dark spots.

Newton's Ring Experiment:

1. In this experiment, a flat glass plate is kept below a spherical lens to obtain an interference pattern using the reflection of light.
2. A thin air film with a definite thickness is produced between the lens and the plate. Thus, the path difference between the incident ray and the reflected ray at center is $0$.
3. But because of reflection, there occurs a phase difference of $180°$. So, the path difference is $\frac{\lambda }{2}$. Hence, destructive interference is produced at the center. Therefore, a dark fringe is obtained.
4. Below in figure (a), is the set-up for the experiment and in figure (b), is the pattern obtained.

Newton's Ring Experiment

Hence, the central fringe is dark in Newton's ring.