It was observed by Hertz in 1887, quite by accident, that a spark would jump easily between two charged spheres when their surfaces are illuminated by light from another spark.
The photoelectric effect, which is the emission of electricity from metals due to incident electromagnetic radiation, was first investigated in detail by Hallwachs & Lenard during 1886-1900.
The explanation of these experimental results came only after Max Planck proposed the quantum theory of radiation.
It was Sir Isaac Newton who had initially proposed the corpuscular theory of light. His theory was abandoned in favour of the wave theory, proposed by Huygens, as the latter was in agreement with experiments like interference and diffraction.
More than a century later, Planck’s quantum theory (somewhat similar to Newton’s corpuscular theory), got support from Einstein in the explanation of the photoelectric effect
According to Planck’s quantum theory, light consists of packets of energy, referred to as photons hereafter, which have the following properties:
(i) A photon of light of frequency ν contains energy E which is directly proportional to the frequency : E = hν, where h is Planck’s constant
(ii) Photons also carry momentum P:
p = E/c = hν/c = h/λ
where E is the energy of the photon, and c is the velocity of light in vacuum.
(iii) A photon is massless (zero rest mass) and moves with the velocity of light in vacuum (c = 3 x108m/s). It can never be brought to rest.