Question

what is refractive index why does a diamond glitter in light

Answer 1

refractive index is the ratio of speed of light on first median I.e. in air to speed of light in second medium.
n21= c/v
diamond glitter in light because in it total internal reflection takes place.

Answer 2

The value of the constant sin I/sin r for a ray of light passing from air into a particular medium is called refractive index of that medium.
Diamond glitter due to total internal reflection. Hope this help u