Chromatic Dispersion of R G & B Rays
- As the ray crosses the boundary into the glass each wavelength bends towards the normal (the dotted green line) by a different amount.
- 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.
Chromatic Dispersion of R, G & B Rays
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About the diagram
Have you already checked out An Introduction to Reflection, Refraction and Dispersion?
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 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 shows an incident ray of light composed of wavelengths corresponding with red, green and blue 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.
- Refraction results in the dispersion of the wavelengths present in the incident ray.
- The table shows the wavelengths and refractive indices for red, green and blue when refracted by crown glass.
- 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 light.
- 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 how to use the refractive index of a medium see: How to Use the Refractive Index of a Medium.
Some key terms
- In optics, a medium is a material through which electromagnetic waves propagate.
- Although electromagnetic radiation is able to propagate through a wide range of media, it is not dependent upon on any medium for propagation and travels at the speed of light through a vacuum.
- The reason an electromagnetic wave does not need a medium to propagate through is because the only thing that is waving/oscillating is the value of the electric and magnetic fields.
- In general terms, empty space (a vacuum) is not considered to be a medium because it does not contain matter.
- It is the permittivity and permeability of a medium that determines how waves travel.
About sections (temp)
Dispersion (or chromatic dispersion) refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.
- Dispersion is the result of the relationship between refractive index and wavelength.
- Every wavelength of light is affected to a different degree by the refractive index of a medium. The result is that each wavelength changes direction and speed by a different amount.
- In the case of white light, the separate wavelengths span out with red at one end and violet at the other.
- A familiar example of dispersion is when white light strikes raindrops and a rainbow of colours becomes visible to an observer.
- The wavelength of an electromagnetic wave is measured in metres.
- Each type of electromagnetic radiation, such as radio waves, visible light and gamma waves, forms a band of wavelengths on the electromagnetic spectrum.
- The visible part of the electromagnetic spectrum is composed of the range of wavelengths that correspond with all the different colours we see in the world.
- Human beings don’t see wavelengths of visible light, but they do see the spectral colours that correspond with each wavelength and the other colours produced when different wavelengths are combined.
- The wavelength of visible light is measured in nanometres. There are 1,000,000,000 nanometres to a metre.
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