Q.
A symmetric double convex lens is cut in two equal parts by a plane perpendicular to the principal axis. If the power of the original lens is 4D , the power of a cut lens will be
Biconvex lens is cut perpendicularly to the principal axis, it will become a plano-convex lens.
Focal length of biconvex lens f1=(n−1)(R11−R21) f1=(n−1)R2(∵R1=R,R2=−R) ⇒f=2(n−1)R…(i)
For plano-convex lens f11=(n−1)(R1−∞1) f1=(n−1)R…(ii)
Comparing Eqs. (i) and (ii), we see that focal length becomes double.
As power of lens P∝focal length1
Hence, power will become half.
New power =24=2D