What is a photon, and how much energy does it carry?
In 1900 Max Planck had found that he could explain the light glowing from hot objects only by assuming that energy is exchanged in small packets. He thought it was a mathematical trick.
In 1905 Albert Einstein, then 26 and working at the patent office in Bern, took the idea seriously. Light itself, he proposed, is made of packets of energy, later called photons. Each photon carries an energy set by its frequency: E = hf, where h is Planck's constant, a tiny number.
The photoelectric effect now makes sense. An electron is freed by one photon, which gives it all its energy at once. If a single photon of red light does not carry enough energy, sending more red photons does not help: each one fails on its own. One photon of blue light is enough.
Einstein received the Nobel Prize in Physics for this explanation. It took a decade for the experiments to convince everyone: in 1916 Robert Millikan, who had set out to prove Einstein wrong, measured exactly what E = hf predicts.
Which photon carries more energy: one of red light or one of blue light? Blue light has a higher frequency, and E = hf. A blue photon (450 nm) carries about 2.8 eV, a red one (650 nm) about 1.9 eV.
A photon is the smallest possible amount of light of a given colour. You cannot have half a photon: light is absorbed and emitted one whole photon at a time. This is where the word quantum comes from: a fixed, indivisible quantity.
E = hf = hc / λ, with h = 6.626 × 10^−34 J·s and c = 3.00 × 10^8 m/s.
A handy shortcut: E in electronvolts ≈ 1240 / λ in nanometres. Red light at 650 nm: 1.91 eV, that is 3.06 × 10^−19 joules.
Ultraviolet photons (below about 400 nm, more than 3 eV) carry enough energy to break chemical bonds in the skin: that is why they cause sunburn and visible light does not.
Light is made of photons: indivisible packets whose energy E = hf depends only on the colour.