Lab Report on Newton's Rings

Title: Newton’s Rings

Objective: To determine the radius of curvature R of a convex lens.

Apparatus: Travelling microscope, plano-convex lens, optical flat, glass plate, sodium lamp and magnifying glass.

Abstract:

The experiment was done to find the radius of curvature of a convex lens using the Newton’s Ring theory. For this we found the diameter for fringes of the interference pattern using the travelling microscope and later plot of graph of dn'+22- d22 /m2 vs n’ which lead us to determine the value of the radius of curvature of the plano-convex lens, R, 551782.6825± 0.024 m. which contained a percentage error of 4.35*10-6 %.

Introduction:

The phenomenon of Newton’s rings, named after Sir Issac Newton, is an interference pattern formed by light incident on the thin film of air between a convex lens and a flat plane. When viewed with monochromatic light, the pattern appears as alternating fringes of dark and bright concentric rings around the point of contact between the plano-convex lens and the adjacent flat.

The rings are formed due to interference between light waves reflected form the top and bottom surfaces of the air film formed between the lens and flat. It can be explained based on the wave theory of light. When light is incident on the upper surface of the lens it is reflected and refracted and when the refracted ray strikes the glass sheet it undergoes a phase change of 180° on reflection at the lower surface. This phase change explains why the contact point between the two surfaces is dark where it would normally be expected to be bright. Depending on the distance between the two surfaces, light waves in the two beams may be in phase, reinforcing each other (constructive interference, bright fringe), or they may be out of phase, canceling each other out (destructive interference, dark fringe). The areas where the waves are in and out of phase occur in a concentric band because the distance between the two reflecting surfaces increase with...