A light ray striking a surface at 90 degrees will:

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

Answer: (A)

When a light ray hits a boundary straight on, it doesn’t bend at all—the transmitted ray continues in the same direction. That’s because the angle of incidence relative to the normal is zero, and Snell’s law makes the transmitted angle also zero. What does change is speed, which depends on the optical density (refractive index) of the medium the light enters. If the second medium is optically denser (higher index) than the first, light slows down as it enters. If it’s less dense, it speeds up. The common scenario described by this choice is entering a denser medium, so the ray slows down. It’s not ignored, and it doesn’t change direction at normal incidence, though some light may reflect at the boundary.