Refractive Index Explained

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The diagram demonstrates the direct relationship between the speed of light as it travels through a vacuum (c) and the speed of light as it travels through any other transparent medium (v).

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Refractive Index Explained

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
How is wavelength affected as light travel from air into glass or water?
The wavelength of incident light decreases as it travels from air into glass or water because they are both optically rare media.
What does Snells’ law deal with?
_x000D_ _x000D_ Snell's law deals with changes in the angle of incidence and angle of refraction as light travels through different media.
Does light bend towards the normal as it crosses the boundary between air and glass or water?
Yes! As light crosses the boundary from a faster medium such as air to a slower medium such as glass or water it bends towards the normal.
How is the refractive index of a medium related to the speed of light?
Refractive index (index of refraction) determines how much slower light travels through a given medium than through a vacuum.

About the diagram

Have you already checked out An Introduction to Reflection, Refraction and Dispersion?

It is the opening page of our Reflection, Refraction and Dispersion Series and contains masses of useful information. This is the table of contents:

Overview of this page

  • This page explains what is meant by the refractive index of a medium (also called the index of refraction).
  • Related terms, including reflection, refraction and chromatic dispersion are covered on earlier pages of this series.
  • An introduction to the law of refraction (sometimes called Snell’s law) also appears in the series.

An overview of refraction

  • Refraction refers to the way that light (electromagnetic radiation) changes speed and direction as it travels from one transparent medium into another.
  • Refraction takes place as light travels across the boundary between different transparent media and is a result of their different optical properties.
  • Refraction is the result of the differences in the optical density of transparent media. Gases have a very low optical density whilst diamonds have a high optical density.
  • When light is refracted its path bends and so changes direction.
  • The effect of refraction on the path of a ray of light is measured by the difference between the angle of incidence and the angle of reflection.
  • As light travels across the interface between different media it changes speed.
  • Depending on the media through which light is refracted, its speed can either increase or decrease.

An overview of refraction and wavelength

  • Every wavelength of light is affected to a different degree when it encounters a medium and undergoes refraction.
  • Every wavelength of light changes both speed and direction by a different amount when it encounters a new medium and undergoes refraction.
  • The change in angle for any wavelength of light undergoing refraction within a specific transparent medium can be predicted if the refractive index of the medium is known.
  • The refractive index for a medium is calculated by finding the difference between the speed of light in a vacuum and its speed as it travels through the medium.
Colour wavelength (nm) Refractive index
Red 640 1.50917
Yellow 589 1.51124
Green 509 1.51534
Blue 486 1.51690
Violet 434 1.52136

The refractive index for crown glass is often given as being 1.52. This table shows how that figure might alter with wavelength

The diagram

  • The refractive index of a medium (sometimes called the index of refraction) is used to calculate the change in speed or direction as light travels from one transparent medium into another.
  • The equation can be applied to any situation where the optical properties of a specific transparent medium are being investigated.
  • The refractive index is used in the design, manufacture and use of prisms, lenses, optical tools and optical equipment of all types.
  • The equation in the diagram demonstrates the direct relationship between the speed of light as it travels through a vacuum (c),  the speed of light as it travels through any other transparent medium (v) and the refractive index of a medium (n).
  • Because the speed of light in a vacuum is always the same, the formula can be used to calculate:
    • The refractive index (n) of a medium if the speed of light through the medium (v) is known.
    • The speed of light in a medium (v) if its refractive index (n) is known.
  • The refractive index of a material (n) can also be used to predict the change of direction of a light ray as it crosses the boundary between transparent media (see Snell’s law of refraction).
  • The diagram identifies the symbols commonly used for refractive index (n), speed of light in a vacuum (c) and speed of light of a medium (v).
  • At the bottom of the diagram, the characteristics of optically dense and optically rare media are explained.

Remember:

  • The speed of light in a vacuum is always 299,792 kilometres per second.
  • A vacuum is an empty space, and because there is nothing to obstruct it, light travels through it at its maximum speed.
  • The speed of light in any other medium is less than 299,792 km/sec.
  • In the right conditions, transparent media cause incident light to change direction and to disperse into their component colours.
  • When light is refracted and changes direction, the angle is determined by the refractive index of the medium it enters.
  • Only a narrow range of wavelengths that form the full electromagnetic spectrum are visible to the human eye.
  • The wavelengths that we can see are known as the visible spectrum.
  • The presence of different wavelengths of light around us results in the colours we see in the world.

Refractive index

  • The refractive index (also known as the index of refraction) of a transparent medium allows the path of refracted light through a transparent medium to be calculated.
  • The refractive index is a ratio calculated by dividing the change in the speed of light in a vacuum by its speed as it travels through a specific medium.
  • The refractive index of a medium can be calculated using the formula:

n = refractive index, c = speed of light in a vacuum, v = speed of light in a transparent medium

  • When light travels through a vacuum, such as outer space, it travels at its maximum speed of 299,792 kilometres per second.
  • When light travels through any other transparent medium it travels more slowly.
  • Refractive indices describe the ratio between the speed of light in a vacuum and the speed of light in another medium.
  • Most transparent media have a refractive index of between 1.0 and 2.0.
  • Whilst the refractive index of a vacuum has the value of 1.0, the refractive index of water is 1.333.
  • The ratio between them is therefore 1:1.333
  • A simple example of a ratio is of mixing concrete using 1 part of cement to 2 part of sand. The ratio is expressed as 1:2.
  • If we divide the refractive index for light travelling through a vacuum (1.0) by the refractive index for glass (1.333) we find that light travels at 75% of the speed of light in a vacuum.

For an explanation of the refractive index (index of refraction) of a medium see: Refractive Index Explained.

For an explanation of the Law of Refraction see: Snell’s Law of Refraction Explained.

Some key terms

Medium

In physics and optics, a medium refers to any material (plural: media) through which light or other electromagnetic waves can travel. It’s essentially a substance that acts as a carrier for these waves.

  • Light is a form of electromagnetic radiation, which travels in the form of waves. These waves consist of oscillating electric and magnetic fields.
  • The properties of the medium, such as its density and composition, influence how light propagates through it.
  • Different mediums can affect the speed, direction, and behaviour of light waves. For instance, light travels slower in water compared to a vacuum.
  • Examples of Mediums:
    • Transparent: Materials like air, glass, and water allow most light to pass through, with minimal absorption or scattering. These are good examples of mediums for light propagation.
    • Translucent: Some materials, like frosted glass or thin paper, partially transmit light. They allow some light to pass through while diffusing or scattering the rest.
    • Opaque: Materials like wood or metal block light completely. They don’t allow any light to travel through them.
  • The permittivity (electrical response) and permeability (magnetic response) of a medium determine how light interacts with it. These properties influence factors like:
    • Refraction: Bending of light as it travels from one medium to another with different densities.
    • Reflection: Bouncing back of light when it encounters a boundary between mediums.
    • Absorption: Light being captured and converted into other forms of energy (like heat) by the medium.

Related diagrams

References
  • In physics and optics, a medium refers to any material through which light or other electromagnetic waves can travel. It’s essentially a substance that acts as a carrier for these waves.
  • Light is a form of electromagnetic radiation, which travels in the form of waves. These waves consist of oscillating electric and magnetic fields.
  • The properties of the medium, such as its density and composition, influence how light propagates through it.
  • Different mediums can affect the speed, direction, and behaviour of light waves. For instance, light travels slower in water compared to a vacuum.
  • Examples of Mediums:
    • Transparent: Materials like air, glass, and water allow most light to pass through, with minimal absorption or scattering. These are good examples of mediums for light propagation.
    • Translucent: Some materials, like frosted glass or thin paper, partially transmit light. They allow some light to pass through while diffusing or scattering the rest.
    • Opaque: Materials like wood or metal block light completely. They don’t allow any light to travel through them.
optical density

Optical density is a measurement of the degree to which a refractive medium slows the transmission of light.

  • The optical density of a medium is not the same as its physical density.
  • The more optically dense a medium, the slower light travels through it.
  • The less optically dense (or rare) a material is, the faster light travels through it.
  • A vacuum has the least optical density and so light travels through it at a maximum speed of 299,792 kilometres per second.
  • Optical density accounts for the variation in refractive indices of different media.

https://en.wikipedia.org/wiki/Absorbance

Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

  • As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
  • As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from the normal and speeds up.
  • In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
  • A fast (optically rare) medium is one that obstructs light less than a slow medium.
  • A slow (optically dense) medium is one that obstructs light more than a fast medium.
  • The speed at which light travels through a given medium is expressed by its index of refraction.
  • If we want to know in which direction light will bend at the boundary between transparent media we need to know:
  • Which is the faster, less optically dense (rare) medium with a smaller refractive index?
  • Which is the slower, more optically dense medium with the higher refractive index?
  • The amount that refraction causes light to change direction, and its path to bend, is dealt with by Snell’s law.
  • Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.
Wavelength

Wavelength is a measurement from any point on the path of a wave to the same point on its next oscillation. The measurement is made parallel to the centre-line of the wave.

Fast medium

Light travels through different media such as air, glass or water at different speeds.  A fast medium is one through which it passes through more quickly than others.

  • Light travels through a vacuum at 299,792 kilometres per second.
  • Light travels through other media at lower speeds.
  • In some cases, it travels at a speed which is near the speed of light (the speed at which light travels through a vacuum) and in other cases, it travels much more slowly.
  • It is useful to know whether a medium is fast or slow to predict what will happen when light crosses the boundary between one medium and another.
  • so:
  • If light crosses the boundary from a medium in which it travels fast into a material in which it travels more slowly, then it will bend towards the normal.
  • If light crosses the boundary from a medium in which it travels slowly into a material in which it travels more quickly, then the light ray will bend away from the normal.
  • In optics, the normal is a line drawn in a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
Refractive index

The refractive index of a medium is the amount by which the speed (and wavelength) of electromagnetic radiation (light) is reduced compared with the speed of light in a vacuum.

  • Refractive index (or, index of refraction) is a measure of how much slower light travels through any given medium than through a vacuum.
  • The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
  • The refractive index of a medium is a numerical value and is represented by the symbol n.
  • Because it is a ratio of the speed of light in a vacuum to the speed of light in a medium there is no unit for refractive index.
  • If the speed of light in a vacuum = 1. Then the ratio is 1:1.
  • The refractive index of water is 1.333, meaning that light travels 1.333 times slower in water than in a vacuum. The ratio is therefore 1:1.333.
  • As light undergoes refraction its wavelength changes as its speed changes.
  • As light undergoes refraction its frequency remains the same.
  • The energy transported by light is not affected by refraction or the refractive index of a medium.
  • The colour of refracted light perceived by a human observer does not change during refraction because the frequency of light and the amount of energy transported remain the same.
Speed of light

The speed (or velocity) of a light wave is a measurement of how far it travels in a certain time.

  • The speed of light is measured in metres per second (m/s).
  • Light travels through a vacuum at 300,000 kilometres per second.
  • The exact speed at which light travels through a vacuum is 299,792,458 metres per second.
  • Light travels through other media at lower speeds.
  • A vacuum is a region of space that contains no matter.
  • Matter is anything that has mass and occupies space by having volume.
  • When discussing electromagnetic radiation the term medium (plural media) is used to refer to anything through which light propagates including empty space and any material that occupies space such as a solid, liquid or gas.
  • In other contexts empty space is not considered to be a medium because it does not contain matter.
Nanometre

A nanometre is a unit of measurement of the wavelength of electromagnetic radiation.

Law of refraction

As light crosses the boundary between two transparent media, the law of refraction (Snell’s law) states the relationship between the angle of incidence and angle of refraction of the light with reference to the refractive indices of both media as follows:

When electromagnetic radiation (light) of a specific frequency crosses the interface of any given pair of media, the ratio of the sines of the angles of incidence and the sines of the angles of refraction is a constant in every case.

  • Snell’s law deals with the fact that for an incident ray approaching the boundary of two media, the sine of the angle of incidence multiplied by the index of refraction of the first medium is equal to the sine of the angle of refraction multiplied by the index of refraction of the second medium.
  • Snell’s law deals with the fact that the sine of the angle of incidence to the sine of the angle of refraction is constant when a light ray passes across the boundary from one medium to another.
  • Snell’s law can be used to calculate the angle of incidence or refraction associated with the use of lenses, prisms and other everyday materials.
  • When using Snell’s law:
    • The angles of incidence and refraction are measured between the direction of a ray of light and the normal – where the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
    • The wavelength of the incident light is accounted for.
    • The refractive indices used are selected for the pair of media concerned.
    • The speed of light is expressed in metres per second (m/s).

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