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Refractive Index

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

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

    If light travels at 2 x 10^8 m/s in glass, the refractive index of glass is 1.5.

  4. 4

    Avoid this incomplete answer

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

Definition

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

Example

If light travels at 2 x 10^8 m/s in glass, the refractive index of glass is 1.5.

Rule to remember

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

Memory hook

Higher n means slower light and stronger bending.

Examples and method

Worked example

If light speed in glass is 2 x 10^8 m/s and in vacuum is 3 x 10^8 m/s, n = c/v = 3/2 = 1.5.

Method to apply

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

Diagram support

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

How CBSE asks it

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

Avoid common mistakes

Common confusion

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

Common wrong answer

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

Exam tip

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

Quick check

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

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

Answer writing and exam use

1-mark answer

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

2-mark answer

Refractive index tells us how much a medium slows down light compared with vacuum or air, and how strongly it bends light. Refractive index n = c/v for absolute refractive index, and for a given pair of media n = sin i / sin r. If light travels at 2 x 10^8 m/s in glass, the refractive index of glass is 1.5.

3-mark answer

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. Refractive index n = c/v for absolute refractive index, and for a given pair of media n = sin i / sin r. If light speed in glass is 2 x 10^8 m/s and in vacuum is 3 x 10^8 m/s, n = c/v = 3/2 = 1.5. You may be asked to calculate n from speed values, compare media, or relate a larger n to bending. Treating refractive index as a speed itself. It is a ratio, not the speed of light in the medium.
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