Which type of light source emits waves that are in phase and have a constant phase difference?

A coherent light source is characterized by emitting waves that are in phase with each other, maintaining a constant phase difference over time. This coherence is essential for a variety of optical phenomena, such as interference and diffraction, which occur when multiple light waves overlap. In coherence, the relationship between the waves’ phases remains stable, allowing for predictable and observable patterns.

For example, lasers are typically considered coherent light sources because they produce light waves that oscillate in unison, making them ideal for applications like holography or precise measurements.

Other types of light sources, such as incoherent light sources, produce waves with random phase relationships, resulting in a loss of consistent interference patterns. Unpolarized light sources emit waves that are oriented in different directions, while monochromatic light sources refer to waves of a single frequency or color; however, they do not inherently ensure coherence.

Thus, the coherence of the waves and the constancy of their phase difference in a coherent light source make it the suitable choice for this question.