RAI SAHEB BHANWAR SINGH COLL EGE NASRULLAGANJ

subject –physics Present To Present By Gyan Rao Dhote Shivani Chouhan

NEWTON ‘S RINGS

Newton’s rings are the fringes obtained due to interference through a film of very able thickness by the division of amplitude of incident wavefront. When a plano – convex lens of large reduse of curvature is placed on a plane glass plate such that the convex survace of the lense is in contact with the plate, a wedge shaped air film is formed between the lens and the plate . The thickness of the film increases out wards form the point of contact. The thickness of film is zero at the point of contact of the lens and the plate. When light from a monochromatic extended source is made incident normally on the film and the film is seen in the reflected part through a microscope, we get a dark spot at the centre surrounded with the alternate cocentric bright and dark rings.

DIAGRAM OF NEWTON’S RINGS

EXPERIMENTAL ARRANGEMENT

Light rays from the source S fall on a plane glass plate P’ kept inclined at un angle 45 with the vertical. The light reflected from the plate P’ falls normally on the plano-convex lens L placed on a plane glass plate P. The curved surface of lens and the upper surface of plate behave respective as the upper and lower surfaces of the air film. Interference occurs between the waves obtained due to partial transmission from the upper and lower surfaces of the film. The interference fringes are seen in the reflected part through the microscope M focused on the film.

DIAMETERS OF DARK AND BRIGHT RINGS

DARK AND BRIGHT RINGS

FORMATION OF NEWTON’S RINGS

AB is the light ray incident normolly on the curved surface of the lens L which is partialy reflected as BC and then partially transmitted from the upper surfase CG of the lens CD.

The light ray BE obtained due to partial transmission from the surface BF of the lens is partiaiiy reflected as EF from the upper surface of the glass plate and then emerges as GH after refrection through the lens.

Thus we get CD and GH as the coherent inter fering rays in the reflected part.

The path diffrence in these interfering rays is

Δ=2μt cos(r+Ɵ)+λ/2

NEWTON’S RINGS IN THE REFLECTED LIGHT

If the thikness of film is zero at the point of contect of the lens and the plate the center point is dark because the path differenc in the interfering rays obtained in the reflected part due the partial reflection and partial transmission once from each of the upper and the lower surfaces of the

film is λ/2 which is the condition of destreuctive interference .

NEWTON RINGS OF REFLECTED LIGHT

NEWTON’S RING IN THE TRANSMITTED LIGHT There are the following two man differences in the

interference frings seen in the reflected and transmitted part

1. The rings in the reflected part are complementary to the rings in the transmitted part the central ring in the reflected part is dark while the central ring in the transmitted part bright and corresponding to each bright or dark ring in the reflected part , there is a dark or a bright ring in the transmitted part

2. 2. The contrast in rings in the transmitted part is very small in comparison to that in the reflected part the reason is that the amplitudes of the inerfering rays in the reflected part nearly eqquel (1:t²=1:0.96) while there is larg difference in the amplitude of the interfering rays in the transmitted part. (1:t²=1:0.04 )

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