45. 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 :

45. 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) \( f_1 + f_2 \)

(b) \( -f_1 + f_2 \)

(c) \( 2f_1 + f_2 \)

(d) \( -2f_1 + f_2 \)

Answer :  (c) \( 2f_1 + f_2 \)