What is the photoelectric effect? Explain it.

(N/A) Photoelectric effect: When radiation with a certain minimum frequency $v_{0}$ strikes the surface of a metal,electrons are ejected from the metal surface. This phenomenon is called the photoelectric effect. The emitted electrons are called photoelectrons.
Equipment for studying the photoelectric effect: Light of a particular frequency strikes a clean metal surface inside a vacuum chamber. Electrons are ejected from the metal and are counted by a detector that measures their kinetic energy.
The results observed in this experiment were:
$(i)$. The electrons are ejected from the metal surface as soon as the beam of light strikes the surface,i.e.,there is no time lag between the striking of the light beam and the ejection of electrons from the metal surface.
$(ii)$. The number of electrons ejected is proportional to the intensity or brightness of light.
$(iii)$. For each metal,there is a characteristic minimum frequency,$v_{0}$ (also known as threshold frequency),below which the photoelectric effect is not observed. At a frequency $v > v_{0}$,the ejected electrons come out with certain kinetic energy. The kinetic energies of these electrons increase with the increase of the frequency of the light used.
The above observations cannot be explained by the electromagnetic wave theory. According to this theory,since radiations were continuous,it should be possible to accumulate energy on the surface of the metal,irrespective of its frequency,and thus,radiations of all frequencies should be able to eject electrons.
Similarly,according to this theory,the energy of the ejected electrons should depend upon the intensity of the incident radiation.