Spherical Surfaces & Thin Lenses
A convex lens of focal length 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 . The photoelectric current now is:
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Answer: (d)
- Photocurrent depends on total light power reaching the cell.
- Light collected depends on the lens aperture area, which is the same for both lenses.
- A shorter makes a smaller, brighter image, but the total power on the cell is unchanged.
- So the current stays , answer (d).