medium importancemedium difficulty5 min

Refractive Index

Refractive index tells us how much a medium slows down light compared with vacuum or air, and how strongly it bends light.

In one line: Higher n means slower light and stronger bending.

Practice This Concept

Understand it

A higher refractive index usually means light travels slower in that medium. For a given pair of media, refractive index also tells us how much the ray bends at the boundary. In school problems, refractive index is often found using c/v or sin i / sin r.

Example

If light travels at 2 × 10⁸ m/s in glass, the refractive index of glass is 1.5.

Formula and rule

Refractive index n = c/v for absolute refractive index, and for a given pair of media n = sin i / sin r.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

If light speed in glass is 2 × 10⁸ m/s and in vacuum is 3 × 10⁸ m/s, n = c/v = 3/2 = 1.5.

Method you can reuse

1Choose the correct formula.
2Put values in consistent units.
3Divide carefully.
4Interpret whether the medium slows light more.

Sketch it in your answer

Draw a boundary with incident and refracted rays, then mark the angles i and r and show the bending direction.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

If light travels slower in a medium than it does in air, what does the refractive index usually become?

Think first, then show answer

The refractive index becomes greater than 1, because the medium slows light down compared with vacuum or air.

Common wrong answer

Treating refractive index as a speed itself. It is a ratio, not the speed of light in the medium.

Don't mix this up

Students mix up refractive index with density alone. A denser-looking medium is not always the one with the highest optical density.

Exam tip

Use the formula given in the question: c/v for speed problems, or sin i/sin r for angle problems.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

You may be asked to calculate n from speed values, compare media, or relate a larger n to bending.

1
Enough for 1 mark
Refractive index tells us how much a medium slows down light compared with vacuum or air, and how strongly it bends light.
2
Add for 2 marks
Example: If light travels at 2 × 10⁸ m/s in glass, the refractive index of glass is 1.5.
3
Add for 3 marks
A higher refractive index usually means light travels slower in that medium.

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