Looking for classes? Ksquare Career Institute, Bengaluru →
A concave mirror of focal length $f_1$ is placed at a distance of d from a convex lens of focal length $f_2$. A beam of light coming from infinity and falling on this convex lens-concave mirror combination returns to infinity. The distance 'd' must equal
A
$-2f_1 + f_2$
B
$f_1 + f_2$
C
$-f_1 + f_2$
D
$2f_1 + f_2$
Detailed Solution
The parallel beam is focused by the lens at its focus, a distance $f_2$ from the lens.
For the beam to return to infinity the rays must retrace their path, so they must hit the mirror normally.
That happens when the focus of the lens is at the centre of curvature of the mirror, a distance $2f_1$ from the mirror.
$d = f_2 + 2f_1$
For the beam to return to infinity the rays must retrace their path, so they must hit the mirror normally.
That happens when the focus of the lens is at the centre of curvature of the mirror, a distance $2f_1$ from the mirror.
$d = f_2 + 2f_1$