I never intuitively understood why light is made of photons, until now

In 1887, Heinrich Hertz discovered electromagnetic waves. Accidentally, he noticed that shining ultraviolet light on metal made sparks jump further, a phenomenon later called the photoelectric effect. This observation, initially overlooked by Hertz, became crucial with the discovery of electrons.

The photoelectric effect, where light ejects electrons from a metal, propelled Einstein to revolutionize our understanding of light. Initially, the intuition was that more intense light would mean more energetic electrons, leading to a bigger spark. However, experimental findings challenged this simple explanation.

Philip Lenard, Hertz's assistant, devised an experiment to measure the kinetic energy of ejected electrons using a stopping voltage. Contrary to expectations, increasing the intensity of light did not increase the stopping voltage, meaning the electrons did not come out with more kinetic energy. While more electrons were ejected, their individual energy remained the same, posing a significant puzzle.

Lenard proposed the 'trigger hypothesis,' suggesting light only triggers electrons, and their energy is an intrinsic property of the metal, not the light's intensity. However, further experiments showed that the frequency (color) of light *did* affect the kinetic energy of the electrons, with higher frequencies leading to more energetic electrons, and frequencies below a certain threshold yielding no electrons at all, regardless of intensity. This contradicted the trigger hypothesis.

Max Planck, while studying black-body radiation, found that his theories only matched experimental results if energy was transferred in discrete 'quanta' or fixed amounts, rather than continuously. He initially considered this a mathematical trick, proportional to the frequency of light, that he hoped to later discard.

Planck's 'counting trick' worked, but he couldn't make the proportionality constant zero. This meant that the discrete nature of energy transfer was not a mathematical convenience but a physical reality. Planck, however, was uncomfortable with this implication, as it challenged the classical understanding of light as a continuous wave.

Einstein connected Planck's work with Lenard's photoelectric effect results. He proposed that light itself is not continuous but composed of discrete packets of energy called photons, much like 'ice cubes' instead of 'water'. This provided a physical interpretation for Planck's quanta and a reason for the discrete energy transfer.

Einstein's photon theory beautifully explained the photoelectric effect's anomalies. Each electron absorbs one entire photon. If a photon's energy is too low (low frequency), no electron is ejected. If the frequency increases, the photon's energy increases, leading to more kinetic energy for the ejected electron. Increasing intensity simply means more photons, thus ejecting more electrons, but not increasing their individual kinetic energy.

Despite its explanatory power, Einstein's photon theory faced significant resistance. It challenged the well-established wave model of light and was even considered a 'blunder' by some, including Max Planck. Furthermore, Philip Lenard, a Nobel laureate, became a vocal critic of Einstein and his theory due to perceived challenges to his own work and ego.

Robert Millikan, an experimental physicist and initial critic of Einstein's theory, set out to disprove it experimentally. Einstein's photoelectric equation predicted a linear relationship between stopping voltage and light frequency, with a slope directly related to Planck's constant (h/e) and an intercept related to the work function of the metal.

Millikan meticulously experimented for years, plotting the stopping voltage against frequency for various metals. To his surprise, he found a consistent linear relationship with the same slope for all metals. Crucially, the experimentally determined value of Planck's constant from this slope matched the value Planck had derived, validating Einstein's photon model. This was a monumental discovery that ultimately led to Einstein's Nobel Prize in 1921 for his explanation of the photoelectric effect, solidifying the quantum nature of light and shaking the foundations of physics.