Question

3. Newton's rings (circular fringes of equal thickness) reflected at normal incidence from the air gap between a convex spherical piece of glass (radius of curvature R) on top of a plane slab of glass. The two pieces are in contact at the centre, as shown in the figure. observed in monochromatic light are R r (a) Is the reflected light bright or dark at the point of contact? Explain. (b) Show that the dark rings have radii given approximately by is the wavelength of the light and m mR, where A r 0, 1,2,.. Hint: you should use the approximation that /1 - x1 xfor small r. (c) Why are Newton's rings easier to see in reflection than in transmission? (10 тarks)

Answer 1

(a) For the interference of reflected light, at point of contact the air gap between two surface is very small than wavelength of light. Thus, there is no path difference between reflected lights. The phase change is only due to reflection which creates phase difference of 180 degree and result in destructive interference i.e. dark spot at center.

In the transmission light, from energy conservation law, all light is being transmitted and hence at center there will be bright spot.

(b)

Sutpore a ing ) raluy ol to thieknen t, A Prom geometreal theorem, preluet Chord is tatesept tatesedeing eqpal to ta produet seations Sromdes AB X BC DBX BE 2(R)-t]xt 2 2 2 Rt- 42 Since ti caan es Small, we 2R Condlition for dlestruetive er fesence m2 2nt Teferacthe inder, whith is 1 2 22

(c) For the case of transmission, contrast between bright and dark spot is less which makes the transmission mode difficult to see.