The speed of light in vacuuum c 299792458 10 8 300 10 8 m s Index of refraction n c v where v is the speed of light in the material c is the speed of light in vacuum and n is the index of refraction. This value is the speed of light in a vacuum. Light travels at approximately 300000 kilometers per second in a vacuum which has a refractive index of 10 but it slows down to 225000 kilometers per second in water refractive index 13.
This index measures how much the medium alters the course that the light travels through reflection refraction occlusion or other obstruction.
Speed of light in water 3 10⁸ m sec 133. When light passes from one medium material to another it changes speed. One of the most important parameters that measures optical properties of a medium is the index of refraction. Light travels at approximately 300000 kilometers per second in a vacuum which has a refractive index of 10 but it slows down to 225000 kilometers per second in water refractive index of 13.