Energy, Mass and Momentum of Photons

For photons we can determine the importannt particle properties energy, mass and momentum.

Energy

The energy of a photon can be determined using the Photoelectric Effect (see previous chapter). It is:

$$E_{Ph} = h \cdot f$$

$$h$$ = Planck constant, $$f$$ = Frequency of the light

Mass

For the mass $$m_0$$ of a photon the value $$0$$ is assumed in the standard model of elementary particle physics.

Momentum

Photons have a momentum, which can be demonstrated by the following experiment:

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An ideal reflective mirror with very low mass is suspended on a long thread. When it is then hit by an intense flash of light, it is repelled like a pendulum.

The momentum can be calculated using the energy of the photon and the speed of light:

$$p_{Ph} = \dfrac{E_{Ph}}{c} = \dfrac{h \cdot f}{c} =\dfrac{h}{\lambda}$$

$$h$$ = Planck constant, $$c$$ = Speed of light, $$f, \lambda$$ = Frequency and wavelength of light

Sources

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