Final answer:
The refractive index is a scalar quantity that expresses the ratio of the speed of light in a vacuum to its speed in a medium, indicating how much light bends when it enters that medium.
Step-by-step explanation:
The refractive index is a scalar quantity because it only has magnitude and no direction associated with it. It represents the ratio of the speed of light in a vacuum to the speed of light in a medium, giving us an indication of how much the light will bend, or refract, when it enters the medium. This property of the material does not require a direction for its definition and thus is not a vector.
Knowing the refractive index is essential for understanding phenomena such as the bending of light rays when moving from one medium to another, total internal reflection, and the dispersion of light. According to Snell's law, the quantity can be calculated using the incident and refracted angles, and it is used in various optical applications including lenses and prisms. For example, the largest change in the refractive index within the human eye occurs at the cornea, which is critical to visual acuity.