Which statement correctly compares the speed of light?

Which statement correctly compares the speed of light?

• A. The speed of light through space is much greater than 3 x 108 m/s.
• B. The speed of light in glass is faster than the speed of light through space.
• C. The speed of light in glass is slower than the speed of light through space.
• D. The speed of light in glass is much greater than 3 x 108 m/s.

Answer: (C)

Light speed depends on the medium because light slows down when it enters materials with refractive indices greater than 1. The speed in a medium is v = c/n, where c is about 3.0 x 10^8 m/s in vacuum and n is the material’s refractive index. Glass has n around 1.5, so v in glass is roughly c/1.5 ≈ 2.0 x 10^8 m/s, slower than in vacuum. Therefore, the statement that light in glass is slower than light through space is the correct one. Choices claiming glass is faster or much faster than vacuum conflict with v = c/n for n > 1.

Light speed depends on the medium because light slows down when it enters materials with refractive indices greater than 1. The speed in a medium is v = c/n, where c is about 3.0 x 10^8 m/s in vacuum and n is the material’s refractive index. Glass has n around 1.5, so v in glass is roughly c/1.5 ≈ 2.0 x 10^8 m/s, slower than in vacuum. Therefore, the statement that light in glass is slower than light through space is the correct one. Choices claiming glass is faster or much faster than vacuum conflict with v = c/n for n > 1.