The ratio of the speed of light in vacuum to that in a medium is called
refractive index of the medium
The absolute refractive index of a medium is defined as \(n = \dfrac{c}{v}\), where c is the speed of light in vacuum and v its speed in the medium.
Luminous intensity and luminous flux are photometric quantities, and magnification relates image and object sizes.
So the ratio of speed in vacuum to speed in the medium is the refractive index.
Hence, refractive index of the medium.
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