Question

light travels at 1.90×10m /s in a crystals . crystals index of refraction?

Answer 1

Reflection is bouncing off a barrier or interface. Refraction is bending of light's path at the interface between two media. All angles are to be measured from the normal to the surface. The incident angle and the reflected angle are equal. The refracted angle may be larger or smaller than the incident angle.
The index of refraction, n, is a ratio that compares the speed of light in a vacuum to the speed of light in another medium.
n = c/v where c = speed of light in a vacuum = 3x108m/s
The speed of light in all other media is less than in a vacuum. Therefore, n is always larger than one. It may be thought of as the measure of the slowness of light in a medium: the slower light travels in medium, the larger n will be for that medium. Since n divides speed by speed it has no units.
The frequency of light does not change as it enters a different medium, but its wavelength does. Since
v = fL where f = frequency and L = wavelength
then
v1/v2 = L1/L2