recoil

In physics, recoil describes the backward movement of an object when a force is applied, much like a cannon recoils when a shot is fired. This phenomenon is a direct consequence of Newton's third law of motion, which states that for every action, there is an equal and opposite reaction.123

When an atom emits a photon, it recoils to conserve momentum. This recoil causes the emitted photon's energy to be slightly less than the energy difference between the atom's states. Similarly, when a photon is absorbed, the atom gains kinetic recoil energy.1

While this recoil effect is minimal for visible light due to the small recoil energy, it becomes significant for gamma-ray photons emitted by nuclei. However, a phenomenon known as the Mössbauer effect allows for recoilless absorption of gamma rays if the nuclei are embedded in a solid, as the recoil momentum is then absorbed by the entire solid lattice.1