Question

The plane surface of a plano convex lens of focal length 20 cm is silvered and made reflecting, as shown in the figure. Find new focal length of the system.

Answer 1

we we should use combination of power of lens /mirror.
e.g., $P=P_L+P_m+P_L$

$P=2P_L+P_m$

power of mirror is 0 because focal length of plane mirror = ∞ so, power = 1/focal length =0

so, $P=2P_L$

$\implies \frac{1}{f}=\frac{2}{f_L}$

$\implies \frac{1}{f}=\frac{2}{20cm}$

hence, new focal length of system = 10cm