What property of a coherent light source is critical for its definition?

A coherent light source is defined by its ability to emit light waves that are in phase with one another. This means that there is a consistent phase relationship between the waves emitted, allowing for constructive and destructive interference to occur predictably. Coherence is essential for phenomena such as interference patterns, which can be observed in experiments like the double-slit experiment.

When light waves are in phase, they maintain a constant phase difference over time and space, enabling them to reinforce or cancel each other effectively. This characteristic is what makes coherent sources, such as lasers, especially useful in applications like holography, telecommunications, and various scientific measurements.

The other properties mentioned do not define coherence. For instance, emitting light at different wavelengths typically indicates a mixture of colors, which doesn't contribute to the coherence of the light. Brightness and concentration of light are aspects of intensity but do not address the phase relationship necessary for coherence. Polarization describes the orientation of the light waves but is not a requirement for coherence. Thus, the consistent phase relationship is the unique and defining property of a coherent light source.