Photon energy is theenergy carried by a singlephoton. The amount of energy is directly proportional to the photon'selectromagneticfrequency and thus, equivalently, is inversely proportional to thewavelength. The higher the photon's frequency, the higher its energy. Equivalently, the longer the photon's wavelength, the lower its energy.
Photon energy can be expressed using anyenergy unit. Among the units commonly used to denote photon energy are theelectronvolt (eV) and thejoule (as well as its multiples, such as the microjoule). As one joule equals6.24×1018 eV, the larger units may be more useful in denoting the energy of photons with higher frequency and higher energy, such asgamma rays, as opposed to lower energy photons as in the optical andradio frequency regions of theelectromagnetic spectrum.
Photon energy is directly proportional to frequency.[1]where
This equation is known as thePlanck relation.
Additionally, using equationf =c/λ,where
The photon energy at 1 Hz is equal to6.62607015×10−34 J, which is equal to4.135667697×10−15 eV.
Photon energy is often measured in electronvolts. One electronvolt (eV) is exactly1.602176634×10−19 J[3] or, using the atto prefix,0.1602176634 aJ, in theSI system. To find the photon energy in electronvolt using the wavelength inmicrometres, the equation is approximately
since =1.239841984...×10−6 eV⋅m[4] whereh is thePlanck constant,c is thespeed of light, ande is theelementary charge.
The photon energy of nearinfrared radiation at 1 μm wavelength is approximately 1.2398 eV.
AnFMradio station transmitting at 100 MHz emits photons with an energy of about4.1357×10−7 eV. This minuscule amount of energy is approximately8×10−13 times theelectron's mass (viamass–energy equivalence).
Very-high-energy gamma rays have photon energies of 100 GeV to over 1 PeV (1011 to 1015 electronvolts) or 16 nJ to 160 μJ.[5] This corresponds to frequencies of2.42×1025 Hz to2.42×1029 Hz.
Duringphotosynthesis, specificchlorophyll molecules absorb red-light photons at a wavelength of 700 nm in thephotosystem I, corresponding to an energy of each photon of ≈ 2 eV ≈3×10−19 J ≈ 75 kBT, wherekBT denotes the thermal energy. A minimum of 48 photons is needed for the synthesis of a singleglucose molecule from CO2 and water (chemical potential difference5×10−18 J) with a maximalenergy conversion efficiency of 35%.