What is the definition of the index of refraction?

The index of refraction is defined as the ratio of the speed of light in a vacuum to the speed of light in a given medium. However, in this context, it is being understood in the more practical sense of comparing the speed of light in air to that in a medium. This ratio quantifies how much light slows down when it enters a different substance. The higher the index of refraction, the slower light travels through that medium compared to its speed in air (or vacuum in general).

This understanding is fundamental in fields like optics, where different materials have distinct indices of refraction that affect how light bends or refracts as it moves from one medium to another. Therefore, the relationship highlighted in the correct answer accurately represents the key concept of the index of refraction in practical applications.

In contrast, the other options either misidentify the components involved or focus on unrelated aspects. For instance, comparing the speeds of sound to light is not relevant to the concept of refraction, which is strictly concerned with light. Similarly, measurements of color temperature do not relate to the speed of light and the index of refraction, focusing instead on a different area within the study of light and colors.