Explain the following observations using Einstein's photoelectric equation : (a) Photoelectric emission does not occur from a surface when the frequency of the light incident on it is less than a certain minimum value. (b) It is the frequency, and not the intensity of the incident light which affects the maximum kinetic energy of the photoelectrons. (c) The cut-off voltage ($V_0$) versus frequency ($ u$) of the incident light curve is a straight line with a slope $\frac{h}{e}$.