Chromatic Dispersion of White Light

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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).
  • The incident ray of light is refracted 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 the higher refractive index.

Description

Chromatic Dispersion of White Light

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
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.
Refraction refers to the way light changes both direction and speed as it travels from one transparent medium into another.
Yes! Light separates into different colours during the course of refraction.
Yes! When light leaves a vacuum or travels from one transparent medium into another, it undergoes refraction causing it to change both direction and speed.

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

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 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.

Remember:

  • 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 around us.

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.

Some key terms

The angle of reflection measures the angle at which light rebounds from a surface after being reflected.

  • The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • The angle of reflection can be used to understand how light will behave when it interacts with different types of surfaces and objects.

Any material through which an electromagnetic wave propagates (travels) is called a medium (plural media).

Summary

About sections (temp)

The angle of reflection is the angle between the incident light ray and the reflected light ray, both measured from an imaginary line called the normal.

  • According to the law of reflection, the angle of incidence (the angle between the incident ray and the normal) is always equal to the angle of reflection.
  • The angle of reflection is measured between the reflected ray of light and an imaginary line perpendicular to the surface, known as the normal.
  • In optics, the normal is a straight line drawn on a ray-tracing diagram at a 90º angle (perpendicular) to the boundary where two different media meet.
  • If the boundary between two media is curved, the normal is drawn perpendicular to the tangent to that point on the boundary.
  • Reflection can be diffuse (when light reflects off rough surfaces) or specular (in the case of smooth, shiny surfaces), affecting the direction of reflected rays.

The angle of reflection measures the angle at which reflected light bounces off a surface.

  • The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

In physics and optics, a wave diagram uses a set of drawing conventions and labels to describe the attributes of light waves including wavelength, frequency, amplitude and direction of travel.

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