Photoelectric Effect was discovered by
A) Hertz
B) Hallwachs
C) Lenard
D) Millikan

Hint: When electromagnetic waves (visible light is a part of electromagnetic waves’ spectrum) is incident on the metal surface causing ejection of an electron from the metal surface is called the photoelectric effect. This effect was first discovered by a scientist in the year 1887, who was working on radio waves and Albert Einstein explained this effect in 1905 for which he got the Nobel prize in 1921.

Complete step by step solution:
When light (electromagnetic waves) shines on a metal surface then due to the photoelectric effect electrons get ejected from the metal surface. If the metal is connected in series to the circuit which has some potential supply then the effect of ejection of the electron due to the photoelectric effect can be noticed by observing the rise in the current in the circuit.
In 19887 German physicist, Heinrich Rudolf Hertz discovered the photoelectric effect while working on relevant radio waves. Hertz made an observation that when ultraviolet light shines on two metal electrodes with potential difference applied across them, the light changes the voltage at which the sparking takes place.
Wilhelm Hallwachs and Philip Lenard tried to investigate the phenomenon of the photoelectric effect and made the observation that electrons get emitted from the metal surfaces only for the light having a frequency above some critical frequency. Where experimental works of Robert Millikan helped to prove Einstein’s theory which explained the photoelectric effect.

Thus, Option A is the correct answer.

Note: The kinetic energy of the electrons released due to the photoelectric effect depends on the frequency of the incident light and not on its intensity. But the rate of emission of electrons released is proportional to the intensity of the incident light.
Also, the kinetic energy of the electron is the difference between the energy of the incident light and the minimum energy required to free the electron from the surface of the metal called work function.