Why is laser light described as coherent, narrow, and monochromatic?

Laser light is characterized as coherent, narrow, and monochromatic due to the unique interaction between the process of stimulated emission, the laser medium, and the resonant cavity.1

• Coherent: Stimulated emission produces photons that are identical to the stimulating photon, meaning they share the same phase, wavelength, and direction. Because these waves maintain a fixed phase relationship, they are considered coherent.12
• Narrow: The beam is focused into a narrow, collimated path as light reflects back and forth within a resonant cavity. While some divergence occurs due to diffraction, the beam remains significantly more directed than light from conventional sources.1
• Monochromatic: Laser light is limited to a very narrow range of wavelengths. This occurs because the laser medium supports stimulated emission only over specific energy transitions, and the resonant cavity further restricts oscillation to wavelengths that meet precise resonant conditions.1

These properties make laser light ideal for applications requiring precise delivery of energy or information, such as in optical ceramics used for laser windows or active lasing media like ruby and Nd-YAG.34