photon energy (E = hf)
The Planck–Einstein relation, E = hf, ties the energy of a single photon directly to the frequency of its light, with the Planck constant h as the proportionality factor. A photon of higher-frequency light carries more energy; a photon of lower-frequency light carries less. It is perhaps the single most important little equation in all of early quantum theory, the bridge that joins the wave property frequency to the particle property energy.
Because frequency and wavelength are inversely related, the same relation can be written E = hc/λ: short wavelengths mean high energy, long wavelengths low energy. This is why a single ultraviolet or X-ray photon can break chemical bonds and damage living cells, while a single radio-wave photon, of vastly longer wavelength, is far too feeble to do anything of the sort, no matter how many of them there are.
This one relation underlies a sweep of phenomena. It explains the threshold of the photoelectric effect, sets the energy of the photon emitted in each atomic transition, and lets a measured spectral colour be read directly as an energy gap inside an atom. Whenever light and matter trade energy quantum by quantum, the bookkeeping is done by E = hf.
Higher frequency means a more energetic photon — energy and colour are two sides of one coin.
Brightness sets how many photons arrive, not how much energy each one has. A blinding red lamp still delivers only low-energy photons, which is why it cannot do what even a faint ultraviolet source can.