Refractive index

Light slows down when it enters glass or water. The more it slows, the more it bends. For light going from medium 1 into medium 2, the refractive index of medium 2 with respect to medium 1 is n21 = v1/v2. Here v1 and v2 are the speeds of light in the two media.

When medium 1 is air or vacuum, the ratio is called the absolute refractive index: n = c/v. It has no unit, because it is one speed divided by another. The medium with the larger value is optically denser. Light going into it bends towards the normal.

A small example

The refractive index of water is 1.33. The speed of light in vacuum is 3 × 10⁸ m/s. So the speed in water is v = c/n = (3 × 10⁸ m/s) / 1.33, which is about 2.26 × 10⁸ m/s.

Where marks go

Question 33(B) of CBSE's 2026-27 Science sample paper asks for a definition with the mathematical expression. The 2026-27 Science marking scheme gives 1 mark for defining n21 through the two speeds and writing n21 = v1/v2. The order matters: medium 1's speed goes on top.

Question 32 of the same paper is an assertion-reason question. A coin in water looks less deep than it is, because light bends away from the normal as it leaves water. The scheme marks both statements true, with the reason explaining the assertion.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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