+ A **photon** is a single particle of light. Not a piece of a wave — the whole thing, indivisible. Every kind of [[electromagnetic spectrum|electromagnetic radiation]] is made of them: radio, visible light, [[X-ray]]s, [[gamma ray]]s. What changes across that range is not what the particle is, but how much [[Energy]] each one carries.+ > Not to be confused with the [[Proton]], which is a heavy, positively charged particle in the [[Atomic nucleus]]. A photon has no charge and no rest [[mass]].++ ## Energy is the only dial++ A photon's energy is set by its frequency alone:++ ```+ E = hν = hc / λ+ ```++ h is the Planck constant. The consequences run through all of physics and chemistry:++ | radiation | typical photon energy | what one photon can do |+ |---|---|---|+ | radio | 10⁻⁹ – 10⁻⁵ eV | flip a nuclear spin, nothing more |+ | visible light | ≈ 2 – 3 eV | trigger a molecule in your retina |+ | ultraviolet | 3 – 100 eV | break a chemical bond; damage DNA |+ | [[X-ray]] | 100 eV – 100 keV | strip an inner electron from an atom |+ | [[gamma ray]] | 100 keV – TeV and beyond | eject nucleons; make matter out of nothing |++ This is why *brightness cannot substitute for colour*. A blinding red lamp will never cause sunburn, and a faint ultraviolet lamp will. Each photon acts alone, so if one photon lacks the energy to do the job, a trillion of them still cannot.++ ## Properties++ | property | value |+ |---|---|+ | electric charge | 0 |+ | rest mass | 0 (experimentally, below any measurable limit) |+ | spin | 1 |+ | speed in vacuum | c, exactly 299 792 458 m/s |+ | antiparticle | itself |++ Because a photon has no rest mass, it has no rest frame. It always travels at c, in every reference frame, and this refusal is the observation that special relativity was built to explain.++ ## Photons are not conserved++ Unlike [[Electron]]s or nucleons, photons can be freely created and destroyed. An atom drops to a lower energy level and one appears. It hits a detector and it is gone entirely — not slowed, not dimmed, gone, with all its energy handed over at once.++ Above **1.022 MeV** a photon can convert into an [[Electron]] and a [[Positron]] — real matter, made from light, at the exchange rate E = mc². Below that threshold it simply cannot, no matter how many photons you gather, because that is twice the electron's rest energy of 510.999 keV.++ ## Where the idea came from++ Classical physics said light was a wave, and it had excellent reasons: interference, diffraction, polarisation. But the wave picture predicted that a dim blue lamp should knock electrons out of metal while a bright red lamp should not — and it could not say why. In 1905 [[Albert Einstein]] proposed that light arrives in discrete packets of energy hν. That solved the photoelectric effect and is what his 1921 Nobel Prize was actually for, not relativity.++ Light is both, and neither picture is a metaphor for the other. Which one you need depends on which question you ask.++ See also: [[gamma ray]], [[Compton scattering]], [[Pair production]], [[Electron]], [[Energy]].
History of Photon
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