Which term describes a beam of photons sharing frequency and phase?

• A. Coherent Light
• B. Monochromatic Light
• C. Fluence
• D. Spot Size

Answer: (A)

Coherence is about how light waves relate to each other in a beam. When photons share the same frequency and stay in a fixed phase relationship, the light is coherent. That fixed phase relation lets the waves stay in step and combine predictably, producing a well-defined, highly directional beam—as seen with lasers. Monochromatic light means the light has a single wavelength, but it doesn’t by itself guarantee a fixed phase relationship among photons. Fluence is just energy per unit area, not about phase or frequency, and spot size is the beam’s diameter on the surface, also not about coherence. So the term that describes a beam of photons sharing frequency and phase is coherent light.