JEE PYQ: Ray Optics Question 9
Question 9 - 2021 (25 Feb Shift 1)
The same size images are formed by a convex lens when the object is placed at 20 cm or at 10 cm from the lens. The focal length of convex lens is ____ cm.
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Answer: (15)
Solution
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$ …(1)
$m = \frac{v}{u}$ …(2)
from (1) and (2) we get $m = \frac{f}{f + u}$
given conditions: $m_1 = -m_2$ $\frac{f}{f - 10} = \frac{-f}{f - 20}$ $f - 20 = -f + 10$ $2f = 30$ $f = 15$ cm
BETA
