CGPET 2017 Solved Paper

The light rays are coming from infinity because parallel rays are incident on the lens. So, from lens formula

1

f

=

1

v

−

1

u

Here u=∞

1

f

=

1

−v

−

1

∞

∴ f=−v
because the final image is formed at optical centre, so the object is at plane mirror
f=u=10cm