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Introduction and derivation of Newtons ring
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Newton's RingNewton's rings is a phenomenon in which an interference pattern is created by the reflection of
light between two surfaces - a spherical surface and an adjacent touching flat surfaces.
Newton's rings phenomenon named so after the name of Sir Isaac Newton, who studied the
effect in 1717. The interference patterns produced in Newton's rings phenomenon are circular in
shape. Therefore, these circular rings are also called Newton's rings. It is to be noted that
Newton's rings is a phenomenon and also the name of interference patterns.
: (i) sketch for coherent sources (ii) Arrangement for Newton's Ring Experiment Newton's rings
phenomenon depends on the interference by reflection in a thin film. The layer of air of varying
thickness between the Plano - Convex lens and a glass plate acts as the thin film. The path
difference between the reflected rays from bottom of Plano - Convex lens and upper surface of
glass plate depends upon the thickness of the air gap between them. As the lens is symmetric
along its axis, the thickness is constant along its circumference of a ring of a given radius.
Hence, Newton's rings are circular in shape.
The apparatus arrangement to produce the Newton's rings consists of a Plano - Convex lens
placed refracted through a convex lens in such a way that the refracted rays are parallel to each
other. These parallel rays then fall upon a glass plate G such that these rays are partially
reflected towards the on a plane glass plate, facing curve surface of lens towards the plate. A
monochromatic light is Plano Convex lens. The glass plate G is so inclined that the rays
reflected from it falls normally on the Plano - convex lens. The interference Phenomenon is
observed from the travelling microscope. When the parallel rays of light fall on the
Plano-Convex lens, the reflection takes place from the lower surface of Plano-Convex lens and
upper surface of its base plate as shown in Fig. 7.10 (ii). The rays reflected from two different
surfaces act as the coherent sources. The superposition of these rays produces the interference
, patterns that can be observed through the travelling microscope. At the point of contact of Plano
- Convex lens and base plate, the thickness of air film is zero. So, there is no geometrical path
difference, but phase is reversed by due to reflection of light at an optically denser medium.
Hence, the center of rings is dark spot. Since the thin air film has symmetrically varying
thickness from center to edge of the Plano-Convex lens, the bright and dark rings of gradually
increasing radii are obtained in the experiment. As the radii of rings increases, the separation of
the rings decreases as shown in Fig. 7.10 (iii)
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