Question

Multi
3.
11.
A double convex lens is made of glass which has refractive index 1.55 for
violet rays and 1.50 for red rays. If the focal length of violet rays is 20 cm
the focal length of red rays is
1) 9 cm
2) 18 cm 3 ) 20 cm
4) 22 cm​

Answer 1

Answer:

4 (22cm)

Explanation:

Answer 2

Answer:

22 cm

Explanation:

Let $f_{R}$ and $f_{v}$ be the focal length of Red and Violet rays respectively.

Formula :- $\frac{1}{f_{v} }$ = (∪ - 1)[$\frac{1}{R_{1} } - \frac{1}{R_{2} }$]

For Violet, by putting the values in the above formula

we get,

=>  $\frac{1}{20}$ = (1.55 - 1)[$\frac{1}{R_{1} } - \frac{1}{R_{2} }$]

=> $\frac{1}{R_{1} } - \frac{1}{R_{2} }$ = $\frac{1}{0.55*20}$

For Red, formula is

=> $\frac{1}{f_{R} }$ = (∪ -1)[$\frac{1}{R_{1} }$- $\frac{1}{R_{2} }$]

putting the values, we getting

=> $\frac{1}{f_{R} }$ = (1.5-1) $\frac{1}{0.55*20}$

        = (0.55×20)/0.5

        = 1.1×20

        = 22cm

Hence, The focal lenght of violet rays is 22cm.

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