Question

A diverging lens of f=20 cm and a converging mirror f=10 cm are placed 5 cm apart coaxially. Where shall an object be placed so that object and its real image coincide?

• A. 60 cm away from lens
• B. 15 cm away from lens
• C. 20 cm away from lens
• D. 45 cm away from lens

Answer 1

Answer: (A)

60 cm away from lens

If the rays are to retrace the path, light ray must fall normal on the mirror. Hence $l$ should be 20 cm from mirror and 15 cm from lens.

$\frac{1}{-15}-\frac{1}{x}=\frac{1}{-20}$

$or\frac{1}{x}=\frac{1}{-15}+\frac{1}{20}=\frac{1}{-60}$

$x=-60cm$ i.e $60cm$ away from lens.