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A photon is an elementary particle. Photons have zero rest mass which allows long distance interactions. The photon is massless, has no electric charge and is stable. A photon has two possible polarization states. Similarly to other elementary particles, photons are best explained by quantum mechanics and wave-particle duality.

A photon moves at the speed of light and its energy and momentum are related by the equation E = pc, where p is the magnitude of the momentum vector p. The energy and momentum of a photon depends only on its frequency or inversely, its wavelength:

E= ℏw=hv= hc/λ

p= ℏk

Where k is the wave vector, w = 2πv is the angular frequency, and ħ = h/2π is the reduced Planck constant.

Photons have applications in modern day technology. The classic photomultiplier tube exploits the photoelectric effect and photons landing on a metal plate. This principal is used in fire detectors. Other detectors use the ability of photons to ionize gas molecules causing a detectable change in conductivity.

Atoms, Energy, and the Periodic Table

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Photon numbers

The entrance exposure during a mammography examination is 100mR. (a) Suppose the photon bean consists of photons each of energy 20 keV. How many photons are incident on a 1 cm^2 surface of breast tissue? (b) Now suppose the energy of each photon is 50 keV. How many photons are incident on a 1 cm^2 surface of tissue?

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Photon Energies and Balmer Series

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