Evidence for a particulate photon?

Problematic evidence for a particle nature of light.

It is commonly accepted that light has a dual particle/wave nature. This is what kickstarted the whole field of Quantum Mechanics. a theory that, in essence, tries to formulate and describe the apparent duality and perceived paradoxicality in our intuition of the universe.

There is overwhelming evidence for the wave-like behavior of light. In this article, however, I want to focus primarily on reasons we think light shows particle-like behavior.

Why is light a particle?

There are a few pieces of supporting evidence for the particle nature of light, the most important of which is the Photoelectric effect. so let’s start with that!

Photoelectric effect:

This is one of the most fundamental mechanisms in our world, making photosynthesis and life on earth possible.

In 1887, German physicist, Heinrich Rudolf Hertz, discovered that ultra-violet light could reduce the voltage required to induce a spark between two metal electrodes. This was before the discovery of the electron itself in 1897!
In 1902, another German physicist demonstrated that light causes electrons to be released from a metal surface.
Apparently, this could not be explained by classical physics, which considered light an electromagnetic wave. The problem was that the kinetic energy of the released electrons did not depend on the intensity of light as it should have based on the wave theory. It was proportional to the frequency of light instead. More light intensity released more electrons, but individual electrons had the same energy.
There was also no lag between the radiation of light and the emission of the electrons.

This is why, in 1921, Albert Einstein, another German physicist, got a noble prize, not because of his groundbreaking theory of general relativity but mainly because of the discovery of the law of photoelectric proposed in a new theory of light:
Each light particle, or photon, has a fixed amount of energy ( Quantum ), depending, inversely, on its frequency (E = hc/λ).