Q. Explain how Newton's ring can be obtained in the laboratory. Why Newton's rings are circular in shape? How the Newton's rings are used to find refractive index of liquid?

Ans. When a Plano-convex lens of large radius of curvature is placed on a glass plate with its convex surface, a thin air film of gradually increasing thickness is formed between the upper surface of glass plate and the lower surface of the Plano convex lens. When this system is viewed in reflected monochromatic light, then alternate bright and dark concentric rings with their dark centre and formed. These fringes are circular in shape because the air film is symmetrical about the point of contact of lens with the plane glass plate. These rings are known as Newton's rings.

These rings are formed because of the interference between the wave reflected from top and bottom surface of an air film formed between the lens and glass plate.

S is broad source of monochromatic light placed at the focus of lens L1. The light rays emitted from source S are made parallel by lens L1. These emitted light rays are incident on glass plate G1 inclined at 45°, rays and rays reflected from this plate falls normally on the system. (Convex lens and plate). Interference occurs between the rays reflected from upper and lower surface of the film. The interference fringes are viewed by a microscope.

Measurement of Refractive Index -

Hence refractive index µ can be determine by measuring the diameter of Newton's rings first with air and then with the liquid for same value of m.