Warm bodies radiate light over a range of frequencies (or wavelengths). This process is called black body radiation. The shape of the frequency spectrum that is produced by black body radiation gave physicists a hard time to explain. Finally Max Planck came up with an idea that worked. He made the assumption that light was radiated in quanta of energy that are proportional to the frequency of the light. The proportionality constant has a very tiny value in term of the everyday units that we use. This constant came to be known as Planck’s constant and it is believed to be a fundamental constant of nature.
The notion that light is radiated (or absorbed) in energy quanta started the whole quantum mechanics revolution. Although there are several other counter intuitive principles associated with quantum mechanics, the relationship between energy and frequency still forms one of the cornerstones of the whole formalism. It naturally leads to a Fourier description of everything in the universe. Frequency is like an indexing label for the Fourier basis functions. As a result one can associate a specific energy with each of these specific basis functions. That then inadvertently leads to the notion of the Heisenberg uncertainty principle, which we have already discussed before.
To reach these conclusions requires a number of inductive steps, mental leaps, which are not necessarily justified. As we have explained before much of the conception baggage associated with quantum mechanics are not confirmed as established experimental facts. Sometimes that is because it is in principle impossible to make such observations. Therefore, it is necessary to review what we actually know for a fact about Planck’s relation.
All Planck did was to assume that light is radiated and absorbed in energy quanta proportional to the frequency. It was Einstein that eventually concluded that light actually exists in terms of these quanta. However, one can explain all phenomena without having to say that light must always exist in terms of these quanta. In fact, this is one of those things that one can never determine experimentally because one can only make observations of light by absorbing it somehow. As a result all that we can make statements about from an observational point of view is what happens during a radiation or absorption process, in other words, during an interaction. So it may well be that light exists as a continuous smooth field as long as it does not interact.
The real question then is: why are radiation and absorption processes quantized?