Grade 10 Science Practice Exam

When light travels from a vacuum, which is the least dense medium, into a denser medium like water or glass, its speed decreases. This phenomenon occurs because light interacts with the particles in the denser medium, leading to a longer effective travel time for the light wave. The denser molecules in these materials cause more frequent interactions, which results in the light wave being absorbed and re-emitted multiple times as it moves through the medium. Consequently, while the speed of light is constant in a vacuum, it is slower in denser substances, leading to a decrease in speed as it transitions from one to the other. This principle is fundamental in optics and is critical for understanding how lenses and other optical devices operate.