Question

An excited atom emits a photon of a certain
frequency. Then two photons of this
frequency fly towards the atom.
Which of the following is likely to happen
next?
1. An atom will emit another photon of the
same frequency.
2. The atom will absorb one of the photons
and will return to its excited state.
3. One of the two photons will knock an
electron out leaving an ion behind.
4. The atom will absorb both photons and
will become more excited than it originally

Answer 1

If a photon of just the right energy hits an atom whose electron is in the excited state, it can induce the electron to jump down to the lower state, emitting another photon, which will be in phase with the incoming photon and traveling in the same direction. This is called stimulated emission.