Question

A symmetric double convex lens in cut in two equal parts by a plane perpendicular to the principal axis. If the power of the original lens was 4 D, the power a cut-lens will be

Options

• 2 D

• 3 D

• 4 D

• 5 D.

Answer 1

2 D

The lens is cut into two equal parts by a plane perpendicular to the principal axis. Thus, the radius of curvature of both the lenses become half of its original value.
As,
\[f = \frac{R}{2}\]
\[P = \frac{1}{f}\]
Radius of curvature becomes half. Therefore, the focal also
reduces to half its original value.
Thus, the power (P) of the two cut-lenses will be equal.
2 P = 4 D
P = 2 D