The refractive index of any medium (air, water, glass, vacuum etc.), is a measure for how much the speed of light (or other waves like sound waves) is reduced inside the medium. For example, glass has a refractive index of 1.5, which means that in glass, light travels at 1/1.5=0.67 times the speed of light in a vacuum.
Two common properties of glass and other transparent materials are directly related to their refractive index.
1. Light rays change direction when they cross the interface from air to the material, an effect that is used in lenses.
2. Light reflects partially from surfaces that have a refractive index different from that of their surroundings.