A symmetric doule convex lens is cut in two equal parts by a plane perpendiculr to the pricipal axis. If the power of the original lens was 4D, the power of a cut lens will be

Options

A. 2D
B. 3D
C. 4D
D. 5D

Answer

A

Explanation

(a) Biconvex lens is cut perpendicularly to the principal axis, it will become a plano convex lens.
Focal length of biconvex lens,
`1/f=(n-1)(1/((R_(1))-1/R_(2))`
`1/f=(n-1)2/R(thereforeR_(1)=R,R_(2)=-R)`
`rArr f=R/2(n-1)`
For p[lano convex lens,
On comparing Eqs. (i) and (ii), we see the focal length become double.
As power of lens, P`prop1/("focal length")`
Hence, power will become half.
New power =`4/2=2D`