The electromagnetic theory of light failed to explain:

$A$ photocell employs the photoelectric effect to convert

The cathode of a photoelectric cell is changed such that the work function changes from $W_1$ to $W_2$ $(W_2 > W_1)$. If the current before and after change are $I_1$ and $I_2$,all other conditions remaining unchanged,then (assuming $h\nu > W_2$):

Light of wavelength $4000 Å$ is incident on a sodium surface for which the threshold wavelength of photoelectrons is $5420 Å$. The work function of sodium is (in $eV$)

During a photoelectric effect experiment,which of the following physical quantities depends on the intensity of the incident radiation?

What is the phenomenon of the photoelectric effect?