The ratio of the speed of light in a vacuum to its velocity in a transmitting medium, such as an optical fiber core.
The ratio between the speed of light through air to the speed of light through another medium; the ratio determines how much a ray of light will bend as it passes through a given medium.
the ratio of the speed of light in a vacuum to its speed in some other medium. This will determine how much light rays are bent. When using immersion objectives it is important to keep the values as close together as possible.