question_answer

To find the focal length of a convex mirror, a student records the following data:

Object pin Convex Lens Convex Mirror Image Pin 22.2 cm 32.2 cm 45.8 cm 71.2 cm The focal length of the convex lens is\[{{f}_{1}}\] and that of mirror is \[{{f}_{2}}.\]

Then taking index correction to be negligibly small, \[{{f}_{1}}\]and \[{{f}_{2}}.\] are close to: [JEE ONLINE 09-04-2016]

A) \[{{f}_{1}}=15.6cm\]                        \[{{f}_{2}}=25.4cm\]

B) \[{{f}_{1}}=7.8cm\]              \[{{f}_{2}}=12.7cm\]

C) \[{{f}_{1}}=7.8cm\]              \[{{f}_{2}}=25.4cm\]

D) \[{{f}_{1}}=12.7cm\]                        \[{{f}_{2}}=7.8cm\]

Correct Answer: D

Solution :

[d] For convex lens

u = - 10 cm

v = 39 cm

For convex mirror