What are Newton's rings and how are they formed?

Newton's rings are a series of concentric circular fringes that are created due to the interference of light waves. This phenomenon occurs when a plano-convex lens is placed on a flat glass surface, forming a thin air film between them. As light waves reflect off the top surface of the lens and the flat surface below, they can interfere with one another, creating bright and dark fringes known as interference patterns.

The bright fringes correspond to constructive interference, where the path difference between the two reflected light waves is an integral multiple of the wavelength, leading to reinforcement of the wave amplitudes. On the other hand, the dark fringes are a result of destructive interference, where the path difference is a half-integer multiple of the wavelength, causing the waves to cancel each other out.

This interference pattern results in a characteristic series of concentric rings that can be observed when viewing the setup through a monochromatic light source, such as a sodium lamp. The rings appear circular due to the symmetry of the air film's thickness decreasing as one moves outward from the point of contact between the lens and the flat surface.