A point source of monochromatic light is initially at a distance d from a photocell, producing a saturation photocurrent. Starting at t=0 , the source is moved away from the photocell at a constant speed v . Which of the following best describes the variation of the saturation photocurrent ( i ) with time ( t )?

Step-by-step solution

The distance of the point source from the photocell at any time t is given by r = d + vt . For a point source, the intensity of light I at a distance r follows the inverse square law: I 1 r^2 . Substituting the expression for r , the intensity at time t is I 1 (d+vt)^2 . The saturation photocurrent i is directly proportional to the intensity of incident light ( i I ). Thus, i 1 (d+vt)^2 . This means the photocurrent decreases non-linearly with time, forming a concave-up curve. Answer: It decreases non-linearly as i