(a) Use conceptual arguments to show that the intensity of light (energy per unit area per unit time) reaching the film in a camera is proportional to the square of the reciprocal of the f -number, as I ∞ 1/(f/D)2
(b) The correct exposure time for a camera set to f/1.8 is (1/500) s. Calculate the correct exposure time if the f-number is changed to f/4 under the same lighting conditions.