First slide

The lens

Question

A double convex thin lens made of glass (refractive index $μ$ = 1.5) has both radii of curvature of magnitude 20 cm. Incident light rays parallel to the axis of the lens will converge at a distance L such that

Moderate

A

L = 20 cm
B

L = 10 cm
C

L = 40 cm
D

L = $\frac{20}{3} cm$

Solution

$\frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}}) Given R_{1} = + 20 cm, R_{2} = - 20 cm, μ = 1 .5 \Rightarrow f = 20 cm . Parallel rays converge at focus. So L = f .$

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