Question

A diverging lens of focal length $f_{1}$ is placed infront of and coaxially with a concave mirror of focal length $f_{2}$ . The separation between the lens and the mirror is $d.$ A parallel beam of light incident on the lens returns as a parallel beam from the arrangement.

• A. The beam diameter of the incident and reflected beams will not be same.
• B. $d=2\left|f_{2}\right|-\left|f_{1}\right|$
• C. If the entire arrangement is immersed in water, the conditions will remains unaltered.
• D. none of the above

Answer 1

Answer: (B)

$\left(d + \left|f_{1}\right|\right)=2\left|f_{2}\right|$
$\Rightarrow d=2\left|f_{2}\right|-\left|f_{1}\right|$