Momentum of light

Advanced

Photons carry momentum even without mass.

Key relationp = h/λ

Photons carry p = E/c = h/λ even with no mass. Absorbing or reflecting light transfers momentum, giving radiation pressure.

Worked example

Solar sails use it.

Common mistake

Momentum doesn't require mass.

In more detail

Light carries momentum, p = E/c, even though it has no mass. Momentum transfer from light is what makes radiation pressure and solar sails work.

Solar sails use radiation pressure from sunlight to propel spacecraft.
Optical tweezers use light momentum to trap and move small particles.
Solar sails are designed to use the momentum of sunlight, which pushes on a large reflective surface in space.

How can light have momentum without mass?

Momentum is linked to energy for massless particles, given by p = E/c.

Is the momentum of light big?

It is very small compared with everyday objects, but it matters for tiny particles and sails in space.

Can light have momentum with no mass?

Yes. Photons have momentum equal to h divided by wavelength, even though they have no rest mass.