Refraction & Dispersion of White Light

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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:

• Introducing reflection
• Types of reflection
• Introducing refraction
• Calculating the angle of refraction
• Introducing chromatic dispersion
• Introducing refraction and wavelength

Overview of this page

• This page looks at the refraction and chromatic dispersion of a ray of white light at the boundary between air and glass.
• Related topics, including reflection, are covered on other pages of this series.
• Introductions to the terms refractive index and the law of refraction (sometimes called Snell’s law) also appear on later pages 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 chromatic dispersion

• The term chromatic dispersion (often simply called dispersion) refers to the way that different wavelengths of light separate at the boundary between transparent media during the process of refraction.
• Dispersion causes the separate wavelengths present in a ray of light to fan out so that their corresponding colours become visible to an observer.
• When white light is dispersed, the spread of colours has red on one side and violet at the other.
• The colours produced by dispersion are spectral colours – ROYGBV.
• Dispersion occurs because refraction causes every wavelength of light to alter speed, and at the same time, to bend and change direction by a different amount.
• For dispersion to occur the incident light approaching the boundary between two different transparent media must contain a sufficiently wide range of wavelengths to enable them to separate out so that their associated colours are visible to an observer.

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.
• To understand dispersion we must recognise that the refractive index of a transparent medium must be corrected for different wavelengths of the visible spectrum.

The diagram

The diagram shows an incident ray of white light approaching the boundary between air and glass.

• As the ray crosses the boundary into the glass it bends towards the normal (the dotted green line).
• Refraction is towards the normal because the ray travels from air, the faster, less optically dense medium with a smaller refractive index into the glass, a slower, more optically dense medium with a higher refractive index.

A familiar example of dispersion is when white light strikes a prism and a rainbow of colours become visible to an observer.

• As light enters a prism it separates into its component wavelengths which an observer perceives as bands of colour.
• Colour is not a property of electromagnetic radiation, but a feature of visual perception experienced by an observer in the presence of different wavelengths of light.

Remember:

• In the right conditions, all 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.
• Refractive index (n) is equal to the speed of light in a vacuum (c) divided by the speed of light in the medium (v)
• Light travels at 299.792 kilometres per second in a vacuum.
• 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 how to use the refractive index of a medium see: How to Use the Refractive Index of a Medium.

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