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This was taken from my Quantum mechanics article.
Albert Einstein, in 1905, published a quantum solution to the photoelectric effect: a phenomenon where photons are shone onto a metal plate and they emit electrons. He also predicted the existence of photons and that light was quantized and existed in these discrete packets of energy.
The next big step for quantum mechanics will be taken by Albert Einstein, in 1905. Albert Einstein was intrigued by the work of Planck and will follow in his lead. Einstein, proposed that light, occurs in individual packets of energy. This idea is now well accepted and this packet of energy is known as the photon.
This proposal was able to explain the photoelectric effect, which is a phenomenon where, light is shone onto a material and electrons are emitted. This is the emission of electricity when light illuminates a charged surface. Einstein predicted that light was made of particles to explain this phenomenon.
The photoelectric effect was first observed by Heinrich Hertz in 1887. Hertz was working to prove James Clerk Maxwell's theory of electromagnetism and in-so-doing, noticed this phenomenon:
Light is shone onto a negatively charged metal plate.
Electrons can be knocked off the surface of the plate.
This is the ejection of electrons from a surface by the action of light.
The problem was observed first in 1902 by Phillip Lenard: the energy of the ejected electrons was proportional to the frequency of the light and not to the intensity. This is a problem because if light was a wave and not composed of particles, there should not be this discrepancy. There was a cutoff frequency, beyond which, electrons were not emitted. The intensity didn't make a difference as the classical theory of light would suggest. Another problem was that there was no observed lag time for the electrons to be ejected. In the classical theory, the electrons would need more time to absorb enough energy to be ejected, especially, if the intensity of the light was more feeble. The electrons would be emitted instantly when light hit the negatively charged surface.
Arthur Compton
Einstein predicted that light was made of particles to explain the phenomenon of the photoelectric effect. This idea will later be proved by the experiments of Arthur Compton in 1923. Compton will prove that photons have momentum. Compton shot a beam of X-rays of a specific frequency at a block of graphite and observed that the scattered radiation was of a lower frequency. According to Compton, this drop in frequency was a consequence of the particle-nature of light. Compton needed a particle description of light to explain it's scattering from electrons. It will also be proposed that photons have no mass since they move at light speed.
This work confirmed the particle-like nature of light.
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