JEE Advance - Physics (2009 - Paper 1 Offline - No. 11)

Using mirror formula for the concave mirror, we have

$${1 \over v} + {1 \over u} = {1 \over f}$$

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

$${1 \over { - 56}} + {1 \over { - 42}} = {{ - 98} \over {56 \times 42}} \Rightarrow f = {{ - 56 \times 42} \over {98}} = - 24$$ cm

Option (B) : When the value is (48, 48), we have

$${1 \over { - 48}} + {1 \over { - 48}} = {{ - 1} \over {24}} \Rightarrow f = - 24$$ cm

Option (C) : When the value is (66, 33), we have

$${1 \over { - 33}} + {1 \over { - 66}} = {{ - 1} \over {22}} \Rightarrow f = - 22$$ cm

Option (D) : When the value is (78, 39), we have

$${1 \over { - 39}} + {1 \over { - 78}} = {{ - 1} \over {26}} \Rightarrow f = - 26$$ 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.