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Published on 04 Nov 2014 | over 2 years ago

Newton’s Rings are nothing but concentric circular alternate bright and dark interference fringes, formed due to superimposition of light beams reflected from a thin air film of varying thickness.


A parallel beam of monochromatic light from an extended source is made incident on a glass plate P (inclined equally to horizontal and vertical) , which reflects it in vertically downward direction. Each ray of light suffers reflections from upper and lower surfaces of thin air film formed between Plano-convex lens and plane glass plate P’. These reflected rays of light superimpose and produce an interference pattern consisting of alternate bright and dark fringes.

As locus of all the points of air film of same thickness, is a circle, so interference fringes due to all rays incident at points corresponding to a particular thickness of air film, merge to appear as a circular ring.

Central fringe is bright and the thickness of fringes goes on decreasing as we move away from central fringe, because thickness of air film goes on increasing as we move away from point of contact of lens with plate P’.

Fringe width varies inversely with thickness of thin film and fringes of varying width are called Fizeau Fringes

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