Question

The distance between a convex lens and a plane mirror is $10$cm. The parallel rays incident on the convex lens after reflection from the mirror forms an image at the optical centre of the lens. The focal length of the lens is $n\times 10$. Find the value of 'n'.

Answer 1

The parallel rays incident on the convex lens converge at the focus,F of the lens. At the mirror, after reflection forms an image at the optical centre of the lens, C.

Thus, the image of the virtual point object Fis formed at C.

$f=CF$

$=CM+MF$

$=10+10$

$=20cm$ the focal length of the lens

Then $n\times 10$ = $20$

$n=2$