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Maharashtra State BoardSSC (English Medium) 10th Standard Board Exam
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Syllabus

Refractive Index

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Topics

  • Introduction
  • Snell’s Law and Refraction in Different Media
  • Absolute Refractive Indices and Applications

Introduction

The refractive index measures how much a light ray bends when entering a different medium due to a change in speed. The refractive index of a medium depends on the velocity of light in that medium and is given by:

`n = "Speed of Light in Vacuum (c)"/"Speed of Light in Medium (v)"`

where:

  • n = Refractive index of the medium
  • c = Speed of light in a vacuum
  • v = Speed of light in the given medium

Since the refractive index is inversely proportional to the velocity of light in a medium:

  • Higher velocity → Lower refractive index (light travels faster).
  • Lower velocity → Higher refractive index (light slows down).

If light moves between two different media, the refractive index is calculated as:

\[\mathrm{Refractive~index~^1n_2=\frac{Velocity~of~light~in~medium~1~(v_1)}{Velocity~of~light~in~medium~2~(v_2)}}\]

If one of the media is a vacuum, the refractive index of the second medium is called the absolute refractive index, denoted by n:

`n = "Speed of Light in Vacuum (c)"/"Speed of Light in Medium (v)"`

For example, the refractive index of water is 1.33, meaning light travels 1.33 times slower in water than in a vacuum.

Snell’s Law and Refraction in Different Media

The refractive index determines how light bends when passing from one medium to another. The laws of refraction are defined by Snell’s Law, which states

  1. The incident ray, refracted ray, and normal all lie in the same plane.
  2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant

\[\frac{\sin\mathrm{i}}{\sin\mathrm{r}}=\mathrm{constant}=\mathrm{n}\]

where:

  • i = Angle of incidence
  • r = Angle of refraction

If a light ray is incident along the normal (i = 0°), it continues in the same direction without bending (r = 0°).

Depending on the medium:

  • From a rarer to a denser medium → Light bends towards the normal.
  • From a denser to a rarer medium → Light bends away from the normal.
  • If light enters perpendicularly, it does not bend.

Refraction of light in different media

Absolute Refractive Indices and Applications

The refractive index varies for different materials. Some common absolute refractive indices are:

Substance Refractive Index Substance Refractive Index Substance Refractive Index
Air 1.0003 Fused Quartz 1.46 Carbon Disulphide 1.63
Ice 1.31 Turpentine Oil 1.47 Dense Flint Glass 1.66
Water 1.33 Benzene 1.50 Ruby 1.76
Alcohol 1.36 Crown Glass 1.52 Sapphire 1.76
Kerosene 1.39 Rock Salt 1.54 Diamond 2.42

Applications of Refractive Index:

  1. It is used in lenses, microscopes, and cameras to focus light.
  2. Helps explain mirages and the bending of light in water.
  3. It is used in optical fibre communication to guide light signals.
  4. Determines the purity of substances like oils and liquids in laboratories.
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Related QuestionsVIEW ALL [86]

What is the unit of refractive index? 

State the law of refraction of light that defines the refractive index of a medium with respect to the other. Express it mathematically. How is refractive index of any medium 'A' with respect to a medium 'B' related to the speed of propagation of light in two media A and B? State the name of this constant when one medium is vacuum or air.

The refractive indices of glass and water with respect to vacuum are 3/2 and 4/3 respectively. If the speed of light in glass is 2 × 108m/s, find the speed of light in (i) vacuum, (ii) water.

How is the refractive index of material related to the velocity of light?

The refractive index of glass with respect to air is `5/4` and the refractive index of water with respect to air is `4/3`. Then what will be the refractive index of glass with respect to water?

Define the term absolute refractive index. The absolute refractive index of diamond is 2.42. What is the meaning of this statement? Refractive indices of media A, B C and D are given below:

Media

Refractive index

A

1.33

B

1.44

C

1.52

D

1.65

In which of these four media is the speed of light (i) minimum and (ii) maximum ?

Find the refractive index of medium C with respect to medium B.

Match the pair:

Group 'A'   Group 'B'
Substance   Refractive index
Air (a) 1.33
  (b) 1.46
  (c) 1.0003

If the absolute refractive index of glass and water are 3/2 and 4/3 respectively, what is the refractive index of glass with respect to water?

Mark the correct answer in the  question.

 If the refractive index of glass with respect to air is 3/2, what is the refractive index of air with respect to glass?

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