Newtons rings

Newton demonstrated interference in the film of air between a convex lens and a flat glass plate. Light reflected at the lower surface of the lens interferes with light reflected at the glass plate, producing a set of alternately bright and dark rings centred on the point of contact between the lens and the glass, as illustrated in Figure 8.12. Newton's measurements showed that the depth of air between the lens and the glass is related to the spacing of the rings. He attempted to explain the phenomenon by using the particle theory of light in conjunction with a convoluted argument.

The interference pattern reflects the circular symmetry of the air film. The rings become progressively closer together towards the edge of the lens, where the thickness of the film

Figure 8.12 Newton's rings.

* Visualization of sound waves using regularly spaced soap films. Eur. J. Phys. 2: 755-765 (2007).

varies more rapidly. Newton's rings are seen to best advantage in monochromatic light.

In the context of Newton's rings, it is the phase difference of 180° between waves reflected at the two surfaces of the air film at the point of contact that gives rise to a dark spot at the centre of the pattern. The film is so thin at that point that it does not introduce a significant additional path difference between the waves.

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