Question

What a lens is cut into two equal halves along the principal axis and perpendicular to principal Axis what will be the focal length of each half

Answer 1

If lens is cut along the principal axis then there will be no change in the focal length of lens. but it is cut perpendicular to principal axis, then focal length will be twice the original one.

Answer 2

If the lens can be cut on the principal axis and then there is no change in length of the focus of lens, but it has been cut perpendicular to principal axis, then focal length is twice that of the original one.

EXPLANATION:

The “focal length of lens” depends on the extending of convergence which it causes.

Focal length of lens calculated using the lens maker formula by

µ is refractive index of material of lens; R1 and R2 are radius of curvature. If one side consider as flat, then Radius of curvature becomes infinity.

Hence focal length f will become

$\begin{array}{c}{\frac{1}{f}=\left(\frac{1}{R_{1}}+\frac{1}{R_{2}}\right)} \\ {\frac{1}{f}=(\mu-1)\left(\frac{1}{R_{1}}\right)}\end{array}$