According to the Planck hypothesis, all electromagnetic radiation is quantized and occurs in finite "bundles" of energy which we call photons. The quantum of energy for a photon is not Planck's constant $h$ itself, but the product of $h$ and the frequency. The quantization implies that a photon of blue light of given frequency or wavelength will always have the same size quantum of energy.
I still don't understand how does a photon look like or how does light/EM waves are quantized.
If in some unbounded system (not in a box, but vacuum with no boundaries), I have "light" and nothing else, and the energy of the system is $E_0$, How do I know how many photons exist in that system?
One way to look at it is: $$ E_0=ℏω_0 $$ it would mean there is one photon with energy of $ℏω_0$. However, one could also see it as
$E_0=2ℏω_1$ where $ω_1=ω_0/2$.
And it would mean that there are two photons with frequency $ω_1$. In other words, how can energy be quantized/discrete when frequency itself is continuous.
I know there are similar post out there but I couldn't really understand it. In some of the post, they mentioned the photoelectric effects, but from what I understood, it just meant that light contains energy, and energy is transferable. There are also answers on Planck hypothesis, for me that is just the mathematical formulation to fit the experimental data. But the intuition behind "light is quantized" is still a mystery to me.