A light ray enters water (n = 1.33) from air (n = 1.00) and slows down. What is its speed in water?

• A. About 3.00×10^8 m/s
• B. About 1.50×10^8 m/s
• C. About 2.0×10^8 m/s
• D. About 2.26×10^8 m/s

Answer: (D)

Light slows down in a medium with a higher refractive index because the electromagnetic waves interact with the material, reducing their phase velocity. The speed of light in a medium is v = c / n, where c is the vacuum speed and n is the refractive index. For water, n ≈ 1.33, so v ≈ (3.00 × 10^8 m/s) / 1.33 ≈ 2.26 × 10^8 m/s. The frequency stays the same across the boundary, so only the speed and wavelength change when light enters water.