Which statement describes the speed of light when entering a slower medium at normal incidence?

Which statement describes the speed of light when entering a slower medium at normal incidence?

• A. Slow down
• B. Speed up
• C. Change direction
• D. Disintegrate

Answer: (A)

When light moves into a slower medium at normal incidence, its speed decreases. The speed in a medium is v = c/n, so if the second medium has a higher refractive index than the first, the light slows down. At normal incidence the light is perpendicular to the boundary, so there is no bending and the direction stays the same; only the speed changes. Therefore, the correct description is that the light slows down. It would not speed up in a slower medium, it would not change direction at normal incidence, and it certainly wouldn’t disintegrate.

When light moves into a slower medium at normal incidence, its speed decreases. The speed in a medium is v = c/n, so if the second medium has a higher refractive index than the first, the light slows down. At normal incidence the light is perpendicular to the boundary, so there is no bending and the direction stays the same; only the speed changes. Therefore, the correct description is that the light slows down. It would not speed up in a slower medium, it would not change direction at normal incidence, and it certainly wouldn’t disintegrate.