Ray Optics

JEE Main level, numerical answer, medium

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Question 40 of 295

The same size images are formed by a convex lens when the object is placed at 20cm20cm or at 10cm10cm from the lens. The focal length of convex lens is

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Answer: 15

m=fu+fm=\dfrac{f}{u+f}
+m=f−10+f....(1)+m=\dfrac{f}{-10+f}....\left(1\right)
−m=f−20+f.....(2)-m=\dfrac{f}{-20+f}.....\left(2\right)
(1)/(2)\left(1\right)/\left(2\right)
−1=f−20f−10-1=\dfrac{f-20}{f-10}
10−f=f−2010-f=f-20
30=2f30=2f
f=15cmf=15cm

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