A point source causes photoelectric effect from a small metal plate. Which of the following curves may represent the saturation photocurrent as a function of the distance between the source and the metal?
As the distance between the source and the metal is increased, the intensity of light 
decreases because
Also, the photocurrent 
is directly proportional to the intensity of light and also to the number of electrons emitted.
Hence, the curve d is the appropriate curve representing above equation.
1