Q 12-11-066JEE MainJEE Main 2024 (27 Jan, Shift 1)Medium
A convex lens of focal length $40\ \text{cm}$ forms an image of an extended source of light on a photoelectric cell. A current $I$ is produced. The lens is replaced by another convex lens having the same diameter but focal length $20\ \text{cm}$. The photoelectric current now is:
Answer: (D) $I$
The photoelectric current depends on the number of photons reaching the cell per second. The lens has the same diameter (same aperture), so it collects the same light energy per second from the source and focuses all of it onto the cell; only the size of the image changes.
The same number of photons strikes the cell, so the current stays $I$.
Solution by Sreeraj P, M.Sc Physics