Photon momentum

Intermediate

Photons carry momentum.

Key relationp = h/λ

A photon has p = h/λ = E/c, even though it is massless. Light exerts pressure and can push atoms (laser cooling).

Worked example

A 500 nm photon: p = 1.3×10⁻²⁷ kg·m/s.

Common mistake

Momentum arises from energy, not mass.

In more detail

A photon has momentum p = h/λ = E/c, even though it has no mass. This momentum is transferred in collisions and radiation pressure.

Light sails use photon momentum to propel spacecraft.
Compton scattering confirmed photon momentum.
Solar sails and laser cooling experiments rely on photons carrying momentum equal to Planck's constant over the wavelength.

How can light have momentum?

Relativity gives massless particles momentum equal to energy divided by c.

Is photon momentum big?

It is tiny for everyday light, but it matters at atomic scales and in space.

Can a massless particle have momentum?

Yes. A photon has no rest mass but still carries momentum because it travels at the speed of light.