What device produces a coherent beam of light?

• A. LED
• B. Sunlight
• C. Laser
• D. Flashlight

Answer: (C)

A laser produces a coherent beam of light. Coherence means the light waves stay in step with each other, having a constant phase relationship over distance and time. This creates a beam that is very narrow and travels in a nearly single direction.

Lasers achieve this through stimulated emission in a gain medium inside an optical cavity. When photons stimulate excited atoms to emit more photons, those new photons have the same phase and direction as the originals. The mirrors in the cavity bounce light back and forth, reinforcing the waves that share the same phase, so the output is a strong, single-wavelength, highly directional beam.

Other options don’t produce this kind of in-phase, tightly directed light. Sunlight is a mix of many wavelengths and random phases, spreading in many directions. LEDs and flashlights emit light in many modes without the strict phase alignment, so their light is not coherent.