Photon energy
Template:Short description Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and thus, equivalently, is inversely proportional to the wavelength. 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 any energy unit. Among the units commonly used to denote photon energy are the electronvolt (eV) and the joule (as well as its multiples, such as the microjoule). As one joule equals Template:Val, the larger units may be more useful in denoting the energy of photons with higher frequency and higher energy, such as gamma rays, as opposed to lower energy photons as in the optical and radio frequency regions of the electromagnetic spectrum.
Formulas
Physics
Photon energy is directly proportional to frequency via the Planck relation.[1]
- Template:Mvar is the photon's energy in J[2]
- Template:Mvar is the photon's frequency in Hz[2]
- Template:Mvar is the photon's wavelength in m
- Template:Mvar is the Planck constant, Template:Physconst
- Template:Mvar is the speed of light in vacuum, Template:Physconst
The photon energy at 1 Hz is equal to Template:Cvt.
Electronvolt
Photon energy is often measured in electronvolts. One electronvolt (eV) is exactly Template:Physconst or, using the zepto prefix, Template:Val, in the SI system. To find the photon energy in electronvolts using the wavelength in micrometres, the equation is approximately since Template:Mvar = Template:Val[3] where h is the Planck constant, c is the speed of light, and e is the elementary charge.
The photon energy of near infrared radiation at 1 μm wavelength is approximately 1.2398 eV.
Examples
An FM radio station transmitting at 100 MHz emits photons with an energy of about Template:Val. This minuscule amount of energy is approximately Template:Val times an electron's mass (via mass–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.[4] This corresponds to frequencies of Template:Val to Template:Val.
During photosynthesis, specific chlorophyll molecules absorb red-light photons at a wavelength of 700 nm in the photosystem I, corresponding to an energy of each photon of ≈ 2 eV ≈ Template:Val ≈ 75 kBT, where kBT denotes the thermal energy. A minimum of 48 photons is needed for the synthesis of a single glucose molecule from CO2 and water (chemical potential difference Template:Val) with a maximal energy conversion efficiency of 35%.