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  What is a photon?

There have been posts and questions about a photon.

Photon is the term in quantum mechanics for one wavelength of electromagnetic radiation.

The number of photons from a photon emitting device is the number of that wavelength it emits. There is no 'particle' involved in the device.

The number of photons counted by a photon detecting device is the number of that wavelength it detects. There is no 'particle' involved in the device.

From Wikipedia:

In [classical] physics, electromagnetic radiation (EM radiation or EMR) refers to the waves of the electromagnetic field, propagating (radiating) through space, carrying electromagnetic radiant energy.
[in classical physics] electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light, which, in a vacuum, is commonly denoted c. In homogeneous, isotropic media, the oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave. The wavefront of electromagnetic waves emitted from a point source (such as a light bulb) is a sphere.

In quantum mechanics, an alternate way of viewing EMR is that it consists of photons, uncharged elementary particles with zero rest mass which are the quanta of the electromagnetic force, responsible for all electromagnetic interactions. Quantum electrodynamics is the theory of how EMR interacts with matter on an atomic level.
Polarization (waves) [is] the ability of waves to oscillate in more than one direction, in particular polarization of light.

photon is the QM term for one wavelength.
Spin is the QM term for the polarization for one wavelength.


Photon has become the universal term rather than saying 'one wavelength' even when not using other QM terms.

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