The refractive index is a measure of how much light slows down when it enters a material compared to its speed in a vacuum. It shows how optically dense a medium is. A higher refractive index means light travels slower in that material. For example, air has a refractive index close to 1, while glass or water has a higher refractive index, meaning light travels slower in them.
In Snell’s Law, the refractive index is used to compare how light bends when passing from one material to another. It helps determine the relationship between the angle of incidence (the incoming light angle) and the angle of refraction (the angle at which light travels in the new medium). If light moves from a material with a lower refractive index to a higher one, it bends toward the normal. If it goes from a higher to a lower refractive index, it bends away from the normal. The amount of bending depends directly on the difference in refractive indices. Thus, the refractive index plays a key role in predicting and understanding the behavior of light as it changes mediums.