JEE Advanced 2018 Physics question
Sunlight of intensity 1.3 is incident normally on a thin convex lens of focal length 20 . Ignore the energy loss of light due to the lens and assume that the lens aperture size is much smaller than its focal length. The average intensity of light, in , at a distance 22 from the lens on the other side is .
Answer: 130
Solution
The parallel beam converges to the focus at 20 cm and then diverges. At 22 cm the beam is 2 cm beyond the focus, so its radius is of the lens
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