Question

In the figure shown the radius of curvature of the left & right surface of the concave lens is 10 cm & 15 cm respectively. The radius of curvature of the mirror is 15 cm.

• A. Equivalent focal length of the combination is 18 cm
• B. Equivalent focal length of the combination is +36 cm
• C. The system behaves like a concave mirror
• D. The system behaves like a convex mirror.

Answer 1

The correct option is C The system behaves like a concave mirror

$P_{eq}=2P_{L1}+2P_{L2}+P_M$

$=\frac{1}{f_{L1}}+2\left(\frac{1}{f_{L2}}\right)-\frac{1}{f_M}$

$P_{eq}=-2\left(\frac{1}{12}\right)+\frac{4}{45}-\left(\frac{2}{-15}\right)$

$P_{eq}=\frac{1}{18}$

$-\frac{1}{f}=\frac{1}{18}$

$f=-18cm$

Hence system act as concave mirror.