Consider data from a series of photoelectric experiments on a
single sample, where the photocurrent 
is measured as a
function of applied voltage V and the brightness B and frequency
of the light source.
Why, for fixed values of V and , are we guaranteed that
always gives the same value regardless of B?
Would you expect this to break down at extremely low illuminations?
If so, how?
Would you expect this to break down at extremely high illuminations?
If so, how?
If we increase the energy of the photons and increase the retarding potential by the corresponding amount, why do we not get the same current?