Energy, Mass and Momentum of Photons

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


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


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


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