Newton’s Rings was discovered by Sir Isaac Newton through his series of experiments regarding interference. He suggested that when a Plano convex lens having a long focal length, placed on a plane glass plane situated optically, a thin air film with varying thickness is enclosed between them. In case of monochromatic light, we can see alternate bright and dark concentric circular rings. They are formed with dark spot at the centre.
The experimental arrangement for producing and observing Newton’s rings is as follows:
The monochromatic source of light is kept at the focus of a condensing lens. The parallel beam of light emerging from the lens falls on the glass kept at an angle of 45O. The glass plate reflects a part of incident light vertically downwards. The reflected beam from the air film is viewed using a mobile casino microscope. One can see alternate bright and dark concentric rings with a dark spot at the centre.
THEORY BEHIND THE RINGS
The formation of Newton’s rings can be briefly explained based on interference between the two waves that are reflected from top and bottom surfaces of the film. If t is the thickness of the air film at a point on it, then the path traced by the refracted wave is 2t.
One of the two reflections takes place at the surface of the denser medium and hence it introduces an additional phase change of or an equivalent path difference λ/2 between two wavelets. An important fact is that,
When Newton Rings are observed in “Mercury vapour lamp” we can see a Concentric Colored ( Rainbow) Rings.