What does the term "coherent" refer to in the context of a laser?

The term "coherent" in the context of a laser refers to light that has the same energy and wavelength, which means that all the light waves are in phase with one another. This uniformity is a defining characteristic of laser light and is what enables lasers to produce a highly focused and intense beam of light.

Coherence is important for various applications of lasers, including interferometry and holography, where the consistent phase relationship between the light waves is crucial for achieving precise measurements and high-quality images. The coherence of laser light allows it to maintain a consistent wavefront, which is essential for applications requiring precision and clarity.

In contrast, light with varying energy and wavelength lacks this uniformity and does not maintain the same phase relationship, leading to a more dispersed beam. Scattered light is a result of interactions with particles or surfaces and does not maintain coherence, while light with different phase angles would not exhibit the uniform behavior associated with coherent light. Therefore, the definition of coherence is directly tied to the parameters of energy, wavelength, and phase, emphasizing the necessity of all these aspects being uniform in laser light.