Which process does a laser use to produce coherent light?

A laser generates coherent light primarily through the process of stimulated emission. In this process, an electron in an excited state of an atom or molecule is induced to fall to a lower energy state by an incoming photon. This photon has a specific energy that matches the energy difference between the excited state and the lower energy state. When this induced transition occurs, the emitted photon has the same phase, frequency, and direction as the incoming photon, resulting in light that is highly organized and coherent.

Coherence in light refers to the uniformity of the wavefronts, which is a hallmark of laser light. Stimulated emission contrasts with spontaneous emission, where photons are emitted randomly and lack coherence. This is why stimulated emission is vital for the operation of lasers, as it amplifies light while maintaining the coherent characteristics necessary for laser applications.

The other processes mentioned do not produce the same coherent light. Reflective scattering involves the bouncing of light off surfaces without the organization needed for coherence. Thermal radiation is related to the random emission of photons due to thermal motion in a body, resulting in a broad spectrum of emitted light rather than coherent light. Diffraction refers to the bending and spreading of waves when encountering an obstacle or aperture, which does not inherently relate to the production of