In optics the refractive index or index o refraction n o a substance (optical medium) is a dimensionless nummer that describes hou licht, or ony ither radiation, propagates throu that medium. It is defined as

${\displaystyle n={\frac {c}{v))}$,

whaur c is the speed o licht in vacuum an v is the speed o licht in the substance. For ensaumple, the refractive index o watter is 1.33, meanin that licht traivels 1.33 times slawer in watter nor it daes in vacuum.