The energy of a photon equals the frequency of the radiation multiplied by Planck's constant. Absorption of photons by atoms and molecules can cause excitation or ionization.
It is usual to suppose that the photon is a stable particle of potentially infinite lifetime. However, as pointed out in a letter from the astronomer Gunnar Welin to New Scientist magazine (20 Oct. 1983), this is a misconception. The true lifetime of the photon is exactly 0 s, thus making it the most unstable particle known. To be meaningful the lifetime must be measured in the reference frame of the particle itself. The photon (as well as the as-yet undetected graviton) moves at the speed of light (of course), hence, no matter how many billions of light-years it has traversed before being caught, for example, in a telescope, the photon itself has not experienced the passage of any time.
The name "photon," which comes from the Greek photos meaning "light," was coined by the American physicist G. N. Lewis in 1926.
Related entries electromagnetic radiation
speed of light
Related category PARTICLE PHYSICS
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