Explain the photoelectric effect.

(N/A) Photoelectric effect: When certain metals (e.g.,potassium,rubidium,caesium,etc.) are exposed to a beam of light,electrons (or electric current) are ejected. This phenomenon is called 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.
Experimental results and observations:
$(i)$ The electrons are ejected from the metal surface as soon as the beam of light strikes the surface. There is no time lag between the striking of the light beam and the ejection of electrons.
$(ii)$ The number of electrons ejected is proportional to the intensity or brightness of light. (Number of electrons $\propto$ Intensity of light).
$(iii)$ Threshold frequency: There is a characteristic minimum frequency,$v_{0}$,below which the photoelectric effect is not observed. This is known as the threshold frequency.
At a frequency $v > v_{0}$,the ejected electrons come out with a certain kinetic energy. The kinetic energies of these electrons increase with the increase in the frequency of the light used.
All the above results could not be explained by the laws of classical physics,but Einstein $(1905)$ was able to explain the photoelectric effect using Planck's quantum theory.
$(\text{Energy of light used}) \propto (\text{Frequency of light}) \propto (\text{Kinetic energy of emitted electron})$ and $(\text{Number of emitted electrons}) \propto (\text{Intensity of light})$.