What defines a laser as being coherent?

Coherence in lasers refers to the property where all the peaks and valleys of the light waves produced move in unison. When a laser is coherent, it emits light waves that are not only in phase with one another but also have a consistent frequency. This means that the light produced is highly ordered, which is crucial for the applications of lasers in areas like communication, medical procedures, and manufacturing.

In contrast, if the light waves had peaks and valleys that traveled independently, as found in incoherent light sources like incandescent bulbs, they would not be able to maintain a consistent phase relationship, leading to a dispersed and less focused beam. Variations in frequency would also imply randomness and a lack of coherence, resulting in poor performance in precision applications. Lastly, if waves were diffused across space, it would further indicate a loss of coherence, as there would be no uniformity in the phase relationship between different parts of the wavefront. Coherence is essential for the unique properties of laser light that make it useful across various fields.