The term photon energy is used for the energy carried
by a single photon of electromagnetic radiation (EMR). A particular photon's
energy is directly related to its wave frequency, i.e.,
the photon is part of an EMR wave such that:
A typical unit of measurement is the electron volt (eV).
For short-wavelength, high-frequency EMR
(gamma rays, X-rays, and often the shorter-wavelength
portion of ultraviolet),
the photon energy (in eV) is generally used to describe the EMR,
whereas wavelength or frequency is more often used for
longer-wavelength EMR.