Refractive index is just a number that tells you how much a medium slows light down compared to air — a bigger number means a bigger bend.
A higher refractive index means light slows down (and bends) more entering that medium.
Light travels fastest in a vacuum (3×10⁸ m/s) and only slightly slower in air, but noticeably slower in water or glass. The refractive index of a medium captures exactly how much slower: it's the ratio of the speed of light in air to the speed of light in that medium, n = c/v.
A higher refractive index means light slows down more in that medium, and also means it bends more when entering it — this is described as the medium being more optically denser. Water has a refractive index of 1.33, while diamond has 2.42 — meaning diamond slows light down (and bends it) far more dramatically than water does.
Here's a common trap: optical density isn't the same as mass density. Kerosene has a higher refractive index than water (it's optically denser), even though kerosene is physically lighter (less mass density) than water.
Key exam points
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Concept of Refractive Index of an Optical Medium — Class 10 Physics · CBSE Class 10 Physics