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Ra Re

refractive index

Ratio of the speed of light in vacuum divided by the speed of light in the current medium (eg, glass).
In other media the light is slower than in a vacuum.Say, the smallest refraction index is 1. 

Glass refracts different wavelengths (“colors”) from different degrees.
Result is different refractive indices, “depending on the light color.”
As a result blue light takes a different path through a lens than for example red or green light!