The refractive index is a measure of how much light slows down when it passes through a material compared to its speed in a vacuum. It is calculated by dividing the speed of light in a vacuum by the speed of light in the material. A higher refractive index indicates that light travels more slowly in that material, which can affect how light bends or refracts when entering or exiting the material. Different materials have different refractive indices. For example, glass typically has a refractive index around 1.5, while water has a refractive index of about 1.33. This property is important in various applications, including the design of lenses in glasses and cameras, as well as in fiber optic technology.