Question

How much intensity of the image is increased if the diameter of the objective of a telescope is doubled?

(a) Two times

(b) Four times

(c) Eight times

(d) Sixteen times

The question was posed to me in quiz.

The origin of the question is Optical Instruments in portion Ray Optics and Optical Instruments of Physics – Class 12

Answer 1

Right choice is (b) Four times

For explanation I would say: On doubling the diameter, the area of the objective increases four times. Its light-gathering power increases four times. The brightness of the image also increases four times. Therefore, the intensity of the image increases by four.