College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 56

(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
(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. Note: " f /4," on a camera, means "an f - number of 4."