What happens to light speed when it enters a denser medium?

• A. It increases.
• B. It remains the same.
• C. It decreases.
• D. It fluctuates randomly.

Answer: (C)

When light enters a denser medium, such as glass or water, its speed decreases. This phenomenon occurs because light travels at different speeds in different materials. In a vacuum, light travels at its maximum speed of approximately 299,792 kilometers per second. However, when it enters a medium that is denser than a vacuum, the interaction between the light waves and the particles of the medium causes the speed of light to slow down.

The extent of this reduction in speed is determined by the refractive index of the medium — a higher refractive index correlates with a greater decrease in speed. This reduction in speed is essential for explaining various optical phenomena, such as refraction, where light bends as it passes from one medium to another. Understanding that light slows down in denser materials is fundamental in fields like optics, photography, and various engineering applications.