What does "coherent" mean in the context of a laser beam?

In the context of a laser beam, "coherent" refers to the property of the light waves being tightly in phase with each other. This means that the peaks and troughs of the waves align consistently over time, resulting in a very focused and monochromatic beam of light. Coherence is a fundamental characteristic that distinguishes laser light from other types of light sources, such as incandescent bulbs or LEDs, which emit light waves that are random in phase and often spread out across a broad spectrum of wavelengths.

The tight phase relationship in coherent light allows for constructive interference, which contributes to the high intensity and directionality of laser beams. This coherence is essential for many of the unique applications of lasers in fields such as medical procedures, communications, and material processing.