"VTU l Physics Cycle l Engineering Physics Lab
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Aim: To determine the radius of curvature of a given plano-convex lens using Newton’s Rings.
Apparatus: Travelling Microscope, light source (LED or sodium vapor lamp), Plano-convex lens, Plane glass plate, Table lamp, reading lens, etc.
Formula : The radius of curvature of a given plano-convex lens:
R=〖〖(D〗_m^2-D_n^2)〗_mean/(4(m-n)"" ) m
where, Dm and Dn are the the diameters of the mth/ nth dark rings (m)
(m-n) is the difference between mth and nth dark ring
 is the wavelength of light source (nm)
Procedure:
The Plano-convex lens and plane glass plate set is placed at the bottom of the microscope. The light source power supply is switched on. The turning glass plate is adjusted so that light falls normally on lens assembly. Newton`s rings are observed and the microscope is adjusted for proper focus, by rotating the drum. The adjustment is made such that center dark fringe at the center of the field of view. Also the cross-wire is adjusted such that their intersection is at the dark spot. The drum of microscope is rotated until the 6th ring on the left-hand side with the cross wire. The micrometer reading is noted and recorded in the table. The cross wire is then made coincides with the 5th ring and the corresponding micrometer reading are noted in the table. The same measurement is continued till the 2nd ring. The cross wire is made coincide with 2nd ring on the right hand side and the corresponding micrometer reading is noted in the table. Similar readings are made for consecutive rings up to the 6th ring on right hand side.
Diameters (D) of the various dark rings are determined and the values of 〖(D〗_m^2-D_n^2) are calculated for mth/ nth dark rings. By taking the average of〖(D〗_(n+m)^2-D_n^2), radius of curvature of plano-convex lens is determined using the given formula.