QJEEM21Ph448a: JEE Main 2021 Physics - Class 12 JEEM21Ph448a Solution: The thickness at the centre of a plane convex lens is 3 mm and…

Question

Verified Solution • 2025–26 Session moderate

The thickness at the centre of a plane convex lens is 3 mm and the diameter is 6 cm. If the speed of light in the material of the lens is 2

\times

10⁸ ms^{$-$ 1}. The focal length of the lens is ____________.

Accepted Answer

{R^2} = {3^2} + {(R - 0.3)^2}

\Rightarrow

{R^2} = 9 + {R^2} + 0.09 - 2 \times 0.3R

\Rightarrow

2 \times 0.3R = 9.09

\Rightarrow

R = 15.15

cm

\mu = {C \over V}

=

{{3 \times {{10}^8}} \over {2 \times {{10}^8}}}

= 1.5

{1 \over f} = (1.5 - 1)\left( {{1 \over R}} \right)

f \simeq 30

cm

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