The refractive index is a measure of how much light bends when it passes from one medium to another, such as from air into water. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium. A higher refractive index indicates that light travels slower in that medium, resulting in more bending. For example, the refractive index of water is about 1.33, meaning light travels 1.33 times slower in water than in a vacuum. This property is crucial in optics, influencing the design of lenses used in glasses, cameras, and microscopes.