Q. What will be effect on Newton's rings if;

(i)      A little oil (µ = 1.65) is introduced between the lens (µ - 1.5) and the glass plate (µ = 1.75).

(ii)    A Plano-convex lens of small radius is used.

(iii)   A plane mirror is placed instead of the glass plate below the Plano-convex lens.

(iv)   The Plano-convex lens is raised by height ?h from the surface of plane glass plate.

(v)    Interference fringes are produced by monochromatic light falling normally on wedge shaped film of cello phase whose refractive index is 1.4. The angle of wedge is 40 seconds and distance between successive fringes is 1.25 mm. Calculate wavelength of the light used.                  [R.T.U. (II Sem.) 2008]

Sol.(a) (I) When a little oil (µ = 1.65) is introduced between the lens (µ = 1.5) and the glass plate (µ = 1.75) as shown in fig. then is this situation both the boundaries i.e. lens to oil and oil to glass plate both are rarer to denser type, so a phase change of π will occur at both the boundaries. In result dark and bright rings (fringes) will interchanged within the area of liquid.

(ii) As we know that diameter of bright and dark ring is directly proportional to the square root of the radius of curvature of the lens. Hence if we use a lens of small radius of curvature, the diameter of the rings will be small and rings will get closer, This will effect on the percentage error in the measurement of diameter.

(iii) If we use plane mirror instead of glass plate the rays transmitted by the air film and striking the mirror will get reflected and no part will be transmitted at all, due to superposition of reflected and transmitted light a general illumination will formed and no fringe will be found i.e. Newton's ring will disappear. Because the condition of maxima and minima for reflected and transmitted light are just opposite to each other.