The index of refraction is defined as which of the following?

• A. The angle at which light bends in a medium
• B. The speed of light in vacuum
• C. The amount that a transparent medium slows down the speed of light through it
• D. The frequency of light in the medium

Answer: (C)

The index of refraction measures how much light slows down in a medium compared with vacuum. It’s defined by the ratio n = c / v, where c is the speed of light in vacuum and v is the speed of light in the medium. This means the higher the n, the slower light travels in that material. That slowdown is what leads to refraction, and the bending angle you observe is determined by how the speeds change between media, described by Snell’s law. The frequency of light stays the same across the boundary, and the speed in vacuum is a fixed constant, not the index itself. So the statement describing the medium’s slowing effect on light’s speed is the best fit.