Quantization and photons

By the early 1900s, 

scientists were aware that some phenomena occurred in a discrete, as opposed to continuous, manner. 

Physicists 

                 Max Planck      and 

                                                     Albert Einstein 

had recently theorized that electromagnetic radiation not only behaves like a wave, 

but also sometimes like particles called photons. 

Planck studied the electromagnetic radiation emitted by heated objects, and he proposed that the emitted electromagnetic radiation was "quantized" since the energy of light could only have values given by the following equation $E_{\text{photon}}=nh\nu$ E​photon​​=nhν, where $n$nn is a positive integer, $h$hh is Planck’s constant—$6.626 \times10^{-34}\,\text {J}\cdot \text s$6.626×10​−34​​J⋅s6, point, 626, times, 10, start superscript, minus, 34, end superscript, space, J, dot, s—and $\nu$ν is the frequency of the light, which has units of $\dfrac{1}{\text s}$​s​​1​​start fraction, 1, divided by, s, end fraction.

As a consequence, the emitted electromagnetic radiation must have energies that are multiples of $h\nu$hν. Einstein used Planck's results to explain why a minimum frequency of light was required to eject electrons from a metal surface in the photoelectric effect.

quantized

When something is quantized, it means that only specific values are allowed, such as when playing a piano. Since each key of a piano is tuned to a specific note, only a certain set of notes—which correspond to frequencies of sound waves—can be produced. As long as your piano is properly tuned, you can play an F or F sharp, but you can't play the note that is halfway between an F and F sharp.