IIT JEE Advanced 2006 Paper 1

For a silvered lens, the equivalent focal length is given by

1

f

=

2

f1

+

1

fm

1

f

=

2

15

+

1

∞

⇒ f =

15

2

A silvered lens acts as an equivalent concave mirror. Applying mirror formula, we get

1

v

+

1

u

=

1

f

⇒

1

v

+

1

−20

=

1

−15 2

⇒

1

v

=

1

20

−

2

15

= −

5

60

⇒ v = −12 cm (to the left of AB) (minus sign indicates that it is to the left of the system.)