Question

The focal length of a biconvex lens is 20 cm. Calculate new focal length if it is cut into two halves as shown in figure.
Please answer urgently

Answer 1

since the focus of the lens is

20 cm

so by lens marker formula

$\frac{1}{f} = \frac{1}{n(\frac{1}{r1} + \frac{1}{r2})}$

since radius of curvature are same for both sides

$\frac{1}{f} = \frac{2}{nR}$

so, $R_{2}$ = infinity for one and $R_{1}$ = infinity for other

then,

$\frac{1}{f'} = \frac{1}{nR}$

$\frac{2}{f'} = \frac{1}{f}$

$f' = 2f$

hence the new focus would be

$20 \times 2 = 40$

so f = 40 cm