QJEEM16Phbf60: JEE Main 2016 Physics - Class 12 JEEM16Phbf60 Solution: To find the focal length of a convex mirror, a student records…

Question

Verified Solution • 2025–26 Session moderate

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 f₁ and that of mirror is f₂. Then taking index correction to be negligibly small, f₁ and f₂ are close to :

Accepted Answer

For lens :

u₁ =

-

(32.2

-

22.2) cm

=

-

10 cm

v₁ = (71.2

-

32.2) cm

= 39 cm

\therefore

{1 \over {{f_1}}}

=

{1 \over {{v_1}}} - {1 \over {{u_1}}}

=

{1 \over {39}}

+

{1 \over {10}}

=

{{49} \over {390}}

\therefore

f₁ = 7.8 cm

For mirror :

R = (71.2

-

45.8) cm

= 25.4 cm

\therefore

f₂ =

{R \over 2}

=

{{25.4} \over 2}

= 12.7 cm

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