Absolute refractive index of a substance is :

Foundation

Physics Foundation

Reflection from spherical mirror

Question

speed of light in water / speed of light in the medium.

speed of light in medium/speed of light in water.

speed of light in air / speed of light in the medium.

speed of light in the medium / speed of light in air.

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The absolute refractive index (n) of a substance measures how much it slows down light. It is defined as the ratio of the speed of light in a vacuum (or air, as a very close approximation) to the speed of light in that medium.

Therefore, the correct formula is: $n = \frac{c_{air}}{v_{medium}}$

Final Answer: speed of light in air / speed of light in the medium.