The phenomenon of the formation of the Newton's rings can be explained on the basis of wave theory of light. When a light ray iP1 from an extended monochromatic light source S is allowed to fall normally on the system (Plano-convex lenses L+ Plane glass plate P), it gets partially reflected a long P1R1 at point P1 and partially transmitted along the path P1P2 and again reflected by upper surface of glass plate along P2R2 at point P2.
There is no phase change at the lens-air surface i.e. at point P1, because the light ray is going from a medium of higher refractive index to lower refractive index. But at the air-plate surface i.e. at point P2 there is a phase shift of π occurs because the reflection take places from a medium of higher refractive index fig (b).
This is the condition of minimum. That is why the centre of Newton's ring in case of reflected light appear dark and appears bright in case of transmitted light because the effective path difference in that case is ?eff→=0 satisfying the condition of maxima.
From equation (ii) for bright fringe the condition must be satisfied is ?eff=n
From equation (v) and (VI), it is clear that a bright or dark ring of any particular order n will occur at a fixed thickness to which remains constant along a circle with its centre at the point of contact. Hence fringes are known as ‘fringes of equal thicknesses.