Practice portal › Dual Nature of Matter and Radiation › Photon Energy, Number of Photons and Intensity

An image of the Sun is formed by a convex lens of focal length 40 cm on the metal surface of a photocell, and a photoelectric current I is produced. If another lens with half the focal length but the same diameter is now used to focus the Sun's image on the photocell, the photoelectric current becomes ______.

Asked in RS Academy GUJCET booklet · Intensity, area and distance

Answer: (3) I

Step-by-step solution

Given: both lenses have the same diameter; the focal length changes from 40 cm to 20 cm.

Idea: the photocurrent is set by the number of photons reaching the metal per second, that is, by the total light power falling on it.

The sunlight collected depends on the lens aperture (its area), which is unchanged.

The image size is proportional to f, so the new image has half the diameter and 1/4 of the area; its intensity is 4 times larger.

But the whole image still falls on the metal, so the total power, and the number of photons per second, is the same.

So the photoelectric current stays I.

Why the other options are wrong

More Photon Energy, Number of Photons and Intensity questionsAll Photon Energy, Number of Photons and Intensity questions →

Reading this solution needs no sign-in. You only sign in to practise against the clock and keep your progress.

Practise this with a timer