JEE Main 2016 · 9 Apr · Shift 1 · Physics Q28

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PhysicsSingle correct+4 / -1Question 28 of 90

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 :

f_1=12.7

cm,

f_2=7.8

f_1=7.8

cm,

f_2=12.7

f_1=7.8

cm,

f_2=25.4

f_1=15.6

cm,

f_2=25.4

Answer and worked solution

Official answer: B

Lens: u = 10 cm, v = 39 cm, so $\frac1{f_1}=\frac1{10}+\frac1{39}$ and $f_1\approx7.8$ cm. With the mirror in place the image pin marks the mirror's centre of curvature: $R=71.2-45.8=25.4$ cm, so $f_2=12.7$ cm.

Topics:Spherical mirrors Thin lenses and lens maker's formula

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JEE Main 2016 · 9 Apr · Shift 1, Physics question 28