Question

why does the red light is the fastest

Answer 1

Light of different wavelengths, or colours, travels at different speeds when they travel through any medium other than vacuum. Red light travels faster than blue in glass because red has higher wavelength
.the light interacts with the material, and those interactions are wavelength dependent. And why Red light travels fasterwhile it has less energy than blue light... However both the lights travel at equal velocity in vacuum or in space.