The Doppler effect exists for more types of waves than just sound waves. This also occurs with light, where a moving observer will see a difference in the frequency of light compared to a stationary observer. This effect occurs when atoms are moving with respect to laser light, which is light of a single frequency.

An atom has an internal structure that can absorb single particles of light, photons, of specific frequencies, but due to the Doppler effect this specific frequency will change when the atom is moving. If the laser light has a lower frequency than the specific absorption frequency of an atom, absorption will only occur for atoms moving towards the laser light. When light is absorbed by the atom moving towards the laser light it will be kicked in the opposite direction of the atom’s motion due to the fact that photons carry momentum, causing the atom to slow down. After a very short time the atom will reemit a photon in a random direction, causing it to be kicked in a random direction. If this process occurs many times then these random kicks from emission events will, on average, cause no change to the motion of the atom, but every absorption event will slow down the atom. The net result is that the atom will slow down over many absorption and emission events. The term ‘cooling’ refers to how this process can slow down a large collection of atoms, which will have an associated temperature. As a collection of atoms slows down their temperature will decrease. Laser cooling can be used to reduce the temperature of trapped atoms to well below 1 Kelvin.