A converging crown glass lens has a focal length 20 cm for violet rays. Its focal length for red rays is ( $μ_{V} =$ 1.56, $μ_{R} =$ 1.53)

20.82 c m

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21.13 c m

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22.85 c m

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24.85 c m

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Solution

The correct option is B $21.13 c m$
$\frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$

$\frac{f_{1}}{f_{2}} = \frac{(μ_{2} - 1)}{(μ_{1} - 1)}$

$f r = \frac{(20) (1.56 - 1)}{(1.53 - 1)}$

$f r = \frac{20 \times 0.56}{0.53} = 21.13 c m$

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A converging crown glass lens has a focal length 20 cm for violet rays. Its focal length for red rays is (μV= 1.56, μR= 1.53)

The correct option is B 21.13cm1f=(μ−1)(1R1−1R2)f1f2=(μ2−1)(μ1−1)fr=(20)(1.56−1)(1.53−1)fr=20×0.560.53=21.13cm

A converging crown glass lens has a focal length 20 cm for violet rays. Its focal length for red rays is ( $μ_{V} =$ 1.56, $μ_{R} =$ 1.53)

The correct option is B $21.13 c m$
$\frac{1}{f} = (μ - 1) (\frac{1}{R_{1}} - \frac{1}{R_{2}})$

$\frac{f_{1}}{f_{2}} = \frac{(μ_{2} - 1)}{(μ_{1} - 1)}$

$f r = \frac{(20) (1.56 - 1)}{(1.53 - 1)}$

$f r = \frac{20 \times 0.56}{0.53} = 21.13 c m$