Newtons Rings

Refractive index of convex Lens

A large focal length (1 metre or more) convex lens L is placed on a glass plate P, kept on the bed plate of microscope,  Rays of light from sodium vapour lamp S, incident horizontally on a glass plate G inclined at 45 ° are reflected vertically downward and are incident normally on the air film enclosed between the lens and glass plate. Due to interference between the light reflected from top and bottom surface of air film, the alternate dark and bright concentric rings can be observed through the microscope. At the point of contact of lens with the plate, the thickness of air film is zero. Therefore, the center of concentric rings appears dark. As one moves away from the point of contact, the thickness of air film increases symmetrically and hence, alternate bright and dark rings are obtained. These rings are called Newton's rings, ( Wiki)

Newtons Rings

Newton's dark rings

Convex Lens, glass plantes, sodium vapour lamp, 45* slot, Vernier microscope.

Newtons Rings

Let the first clear dark ring be nth ring. The microscope is moved slowly to the left side by working its screw to cover, say, twenty seven dark rings. The rings are counted as n, n + 3, n + 6 etc up to n + 27. The vertical cross wire is made tangent to (n + 27) th dark ring and the reading in horizontal scale is noted. By working on horizontal screw of the vernier, the vertical cross wire is made tangent to (n + 24)th (n + 21)th, ... n ring on the left side and respective readings on the scale are noted. Now moving the microscope in same direction, {i.e., moving to the right of the centre to avoid error due to back-lash], the cross wire is made tangential to n(n + 3) rd (n + 6), (n + 27) dark ring on other side (right side) of the center. The corresponding readings are taken and are tabulated as in Table The difference in readings between two sides of a particular ring gives the diameter of that ring. For exam ple dn is the diameter of nth Newton's dark ring.

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