IIT JEE Advanced 2009 Paper 1

Using mirror formula for the concave mirror, we have $\frac{1}{v} + \frac{1}{u}$ = $\frac{1}{f}$ Option (A): When the value is (42, 56), we have $\frac{1}{-56} + \frac{1}{-42}$ = $\frac{-98}{56 \times 42}$ ⇒ f = $\frac{-56 \times 42}{98}$ = * 24 cm Option (B): When the value is (48, 48), we have $\frac{1}{-48} + \frac{1}{-48}$ = $-\frac{1}{24}$ ⇒ f = - 24 cm Option (C): When the value is (66, 33), we have $\frac{1}{-33} + \frac{1}{-66}$ = $-\frac{1}{22}$ ⇒ f = - 22 cm Option (D): When the value is (78, 39), we have $\frac{1}{-39} + \frac{1}{-78}$ = $-\frac{1}{26}$ ⇒ 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.