IIT JEE Advanced 2009 Paper 1

Using mirror formula for the concave mirror, we have

1

v

+

1

u

=

1

f

Option (A): When the value is (42, 56), we have
/−56+

1

−42

=

−98

56×42

⇒ f =

−56×42

98

= * 24 cm
Option (B): When the value is (48, 48), we have

1

−48

+

1

−48

= −

1

24

⇒ f = - 24 cm
Option (C): When the value is (66, 33), we have

1

−33

+

1

−66

= −

1

22

⇒ f = - 22 cm
Option (D): When the value is (78, 39), we have

1

−39

+

1

−78

= −126 ⇒ f = - 26 cm
Since options (A) and (B) give f = −24 cm, which is the same as that given in the problem, we conclude that the options (C) and (D) as the correct options.