You're likely referring to Newton's rings, which are a phenomenon observed when a convex lens is placed on a flat glass surface. These rings appear as a series of concentric, alternating bright and dark circles. Here's why this happens:

Interference of Light Waves

The bright and dark rings are a result of interference between light waves. Here's a breakdown:

1. Thin Film: The space between the lens and the flat surface acts as a thin film of air.

2. Reflection: When light from a single source (like a sodium lamp) strikes the setup, it gets reflected at both the top surface of the air film (lens) and the bottom surface (flat glass).

3. Path Difference: The two reflected beams travel slightly different paths, creating a path difference. This difference depends on the thickness of the air film, which varies with the distance from the point of contact.

4. Interference: The two reflected beams recombine, and their amplitudes interfere.

* Constructive Interference: When the path difference is a whole number of wavelengths, the waves reinforce each other, resulting in a bright fringe.

* Destructive Interference: When the path difference is an odd multiple of half a wavelength, the waves cancel each other out, leading to a dark fringe.

Why Rings?

The rings form because the thickness of the air film increases gradually as you move away from the point of contact. This leads to a gradual change in the path difference, causing the interference pattern to alternate between constructive and destructive interference, creating the rings.

Key Points:

* The diameter of the rings depends on the wavelength of the light used.

* The shape of the lens and the surface also influence the pattern.

* Newton's rings are a classic example of wave interference, demonstrating the wave nature of light.

Let me know if you have any other questions about Newton's rings or interference!