The first hints at the corpuscular (particle) nature of light had come in 1900. Max Planck, a German physicist born in Kiel in 1858, found that he was forced to introduce the idea of discrete packets of light in order to explain
how light or other electromagnetic radiation, is emitted by a hot body. But let's take it from the top. Have you ever been to a blacksmith?

He takes a piece of iron, puts it on the burning coal and waits. The piece of iron will slowly turn red, then yellow, and then white before the blacksmith starts beating it up.
As the piece is heated, electrons, atoms and molecules move or jiggle more rapidly. These movements in the hot body cause the formation of very small "antennas" radiating electromagnetic waves. The theory at that time (Maxwell theory), stipulated that the energy emitted in the form of electromagnetic waves

should be equal at all frequency ranges. Now since the number of frequencies is unlimited (from 0 to infinity), the total radiated energy should be infinite. This is of course a ridiculous prediction which disagrees completely with our observation. Obviously something was wrong ?!

(The ultraviolet catastrophe)

To solve this puzzle, Planck was forced to introduce a revolutionary, simple yet counterintuitive proposal: tiny antennas in the hot body are "quantized", meaning that they can emit electromagnetic radiation only in finite energy quanta (the plural of quantum or "how much" in Latin). The energy E of each quantum is related to the frequency f of the wave by the simple relation

E = hf

where h is the Planck constant. Its value is very tiny, 6.622.10-34 joule.second.
Then at any frequency, energy is emitted by quanta (discrete amounts) of energy hf.
At high enough frequency, the emission of a single quantum (or photon) would require more energy than is available. The radiation at high frequencies would be reduced, and the rate at which body looses energy would be finite. Story of the rich man


Going back to the blacksmith, why does the piece of iron change color when it is heated?We see colors if the frequency (or wavelength) of the emitted electromagnetic wave is in the visible light range. We have just learned that the emitted energy is in the form of discrete quanta called photons. Each photon carries a specific indivisible amount of energy depending only on the frequency of the wave. Now if the hot piece of iron emits a photon corresponding to the frequency of the visible red light, we will see it red. As the temperature gets higher, the emitted photons shall correspond to red and yellow colors,so that the iron will turn orange, and so on...Visit to the blacksmith


Planck's proposal was met with a mixed reception at first. It seemed to explain the electromagnetic radiation of hot bodies pretty well, but only by a kind of mathematical sleight of hand, a trick. Einstein, then an almost unknown physicist, gave that m
mathematical trick a respectable physical reality when he showed that another great puzzle of the time could be explained if light was composed of particles of energy hf. (the photoelectric effect).