Final answer:
The main difference between laser light and ordinary light is that laser light is coherent, meaning its photons are phase linked and it stays narrow over long distances, opposed to the incoherent light emitted from ordinary light sources.
Step-by-step explanation:
The main difference between laser light and ordinary light involves coherence. While both ordinary light and laser light can travel in straight lines and have various colors and frequencies, coherence is what primarily sets laser light apart. Lasers produce light that is not only monochromatic, meaning it contains photons of a single frequency or color, but also emits these photons in a phase linked or coherent manner. This coherence allows laser beams to stay narrow over long distances and makes them useful in applications like CD and Blu-Ray players, where they effectively read digital information stored on discs. In contrast, ordinary light sources, such as a light bulb, emit light that is incoherent; the photons are not in phase with one another and spread out more as the light travels. Therefore, when identifying the unique characteristic of laser light in comparison to ordinary light, coherence is the correct answer.