Q. How shall you measure wavelength of light used in Newton's ring experiment? Derive the formula used. 

Measurement of Wavelength of Monochromatic Light using Newton’s Ring Methods:

Light from monochromatic extended light source is allowed to fall on a glass plate G inclined at angle of 45? to the incident beam and is reflected normally on to a convex lens L of very large radius of curvature placed on a plane glass plate P. As a result concentric alternate bright and dark fringes are formed in the air film between the lens and the glass plate. These fringes are observed by means of a microscope M focused on the air film and their diameters are measured.

Procedure: After obtaining a good contract ring pattern in field of view we have to follow the following steps in order to measure A.

1.    Move the microscope in horizontal direction to bring its crosswire at the centre of the microscope.

2.    Now, counting the rings by moving microscope in one side and set the cross: wire YY’ tangential to one of the rings and note down the reading of microscope.

3.    By moving the microscope towards centre of pattern note down the readings for alternation ring.

4.    Now repeat the same procedure on the other hand for the same number of rings while moving on other side.

5.    From these reading calculate the diameter of each ring.

Now choose a suitable value of m, find average value of D2n+m – D2n and calculate the value of ¥. Value of r i.e. radius of curvature of Plano convex lens can be calculate using speedometer. Same procedure is used for the measurement of reflective index of given liquid.