Which statement best describes how light speed changes with refractive index?

• A. Light travels faster in higher RI materials than in lower RI materials.
• B. Light travels at the same speed in all materials.
• C. Light travels more slowly in higher RI materials than in lower RI materials.
• D. Light slows down only in air.

Answer: (C)

Light slows down as the refractive index increases because n is defined by n = c/v, where c is light’s speed in vacuum and v is its speed in the material. Rearranging gives v = c/n. So a higher n means a smaller v. That’s why light travels more slowly in higher RI materials than in lower RI ones. For example, air has n about 1.0003, water about 1.33, and diamond about 1.52, so light slows progressively more in denser, higher-n media. The statement that light travels more slowly in higher RI materials best captures this relationship. The other options contradict this fundamental link between refractive index and speed: speed isn’t the same in all materials, it doesn’t increase with RI, and it isn’t slowed only in air.