How Does a Laser Work?

A laser works by stimulating atoms or molecules to emit light at specific wavelengths and amplifying that light to produce a narrow beam of radiation. This process of stimulated emission works by imparting energy to atoms or molecules. These particles do not stay in these higher-energy, or excited, states for long as these states are naturally unstable. Dropping back to a base energy state causes atoms to release the extra energy as light. This emission can occur spontaneously or be stimulated by incoming photons (small packets of light).

In a laser, a passing photon with the right energy stimulates an excited atom to emit a photon of the same energy, phase, and direction, creating a coherent light wave. This is possible if more atoms of the laser material are in an excited state than in a lower state, a condition known as population inversion—without it, incoming photons get absorbed instead of multiplying, and a laser cannot be produced. Achieving this requires an external energy source, such as a bright light or electric current, to selectively excite the atoms or molecules. Once a population inversion is established, the emitted photons stimulate further emissions, creating a cascade effect that amplifies the light.

A laser uses a pair of mirrors facing each other—an optical resonator—to build up the light energy in the beam. As light bounces back and forth between the mirrors, it passes through the laser material, increasing in intensity with each pass as more excited atoms emit additional photons. One of the mirrors is partially transparent, allowing some light to escape as the laser beam. This setup ensures that the emitted light is coherent, monochromatic, and focused in a narrow beam.