Chromatic Dispersion in a Prism
Chromatic Dispersion in a Prism
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About the diagram
Have you already checked out An Introduction to Reflection, Refraction and Dispersion?
- When light crosses the boundary between two different transparent media it undergoes refraction.
- The effect of refraction is that light changes speed and its direction of travel.
- The change in speed can be calculated if the refractive index is known.
- The index of refraction can be calculated if the speed of light in a vacuum and the speed of light in the medium is known.
- The refractive index of a medium is calculated using the formula: Where n = refractive index, c = speed of light in a vacuum, v = speed of light in a transparent medium.
- The amount that the path of a ray of light bends when it changes direction is calculated using the Law of refraction (also known as Snell’s law).
Chromatic dispersion is often simply called dispersion.
- Whenever you see a rainbow of colours in a patch of oil, in the edge of a sheet of glass or a crystal, it is caused by dispersion.
- White light, containing all wavelengths of the visible spectrum, disperses into a rainbow of colours.
- Dispersion takes place because the refractive index of any transparent medium is different for each wavelength of light.
- The diagram at the top of the page shows that in certain circumstances when white light strikes a prism, a rainbow of colours become visible to an observer.
- Prisms are generally made from crown or flint glass depending on their intended use.
- Flint glass prisms are often used for experimental purposes to study the refraction and dispersion of light.
- A triangular prism consists of two triangular ends and three rectangular faces.
- If white light is to be refracted or dispersed by a prism into its component colours a narrow beam is pointed towards one of the rectangular faces.
- Dispersive prisms are used to break up light into its constituent spectral colours.
- Reflective prisms are used to reflect light, in order to flip or invert a light beam.
- Triangular reflective prisms are a common component of cameras, binoculars and microscopes.
Crown glass is a type of optical glass made without lead or iron and used in the manufacture of lenses and other tools and equipment concerned with the visible part of the electromagnetic spectrum.
- Crown glass produces low levels of chromatic dispersion which is of particular concern in the manufacture of lenses.
- Dispersion is unavoidable but a well-designed lens is able to reorganize light so that, in the end, all wavelengths converge at the same point and so produce a sharp image with a high degree of colour accuracy.
Flint glass is made from a combination of silicon dioxide (SiO2) and lead or potassium.
- Flint glass typically has a higher refractive index value than crown glass which means that dispersion is more evident.
- Flint glass absorbs most ultraviolet light but comparatively little visible light and is often used in telescope lenses.
- The light source used produces white light which is focused into a narrow beam.
- As the ray enters the prism the angles of incidence and refraction are the same.
- When the light exits the prism the angles of incidence and refraction are the same.
- The light source and prism are arranged on a suitable surface, such as a piece of paper so that the dispersed colours are visible to an observer.
- Remember that light is only visible when either its source is in view or when transmitted light strikes a surface, in this case, the paper.
- The human eye sees white when all the colours that make up visible light are combined together and strike a neutral coloured surface that reflects all wavelengths equally.
- The incident white light is refracted towards the normal as it enters the prism because the optic density of glass is greater than air.
- On entry to the prism, a small amount of dispersion takes place.
- As the dispersed colours exit the prism they are refracted away from the normal because the optic density of air is less than air.
- On exiting the prism, the amount of dispersion of each colour is more pronounced.
- The amount that light bends as refraction and dispersion take place depends on:
- The type of glass.
- The composition of wavelengths produced by the light source.
- The angle of incidence of the light.
- The refractive index of the glass for each wavelength.
Some key terms
- 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 at which incident light from the Sun or a light bulb strikes a surface can affect the outcome. For instance, when incident light hits a mirror, the angle of incidence determines the angle of reflection.
- 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.
- Visible light is a form of electromagnetic radiation.
- Other forms of electromagnetic radiation include radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.
- Visible light is perceived by a human observer as all the spectral colours between red and violet plus all other colours that result from combining wavelengths together in different proportions.
- A spectral colour is produced by a single wavelength of light.
- The complete range of colours that can be perceived by a human observer is called the visible spectrum.
- The range of wavelengths that produce visible light is a very small part of the electromagnetic spectrum.
Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.
- The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
- The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
- Defining rainbow colours is a question more closely related to the relationship between perception and language than to anything to do with physics or scientific accuracy.
- Even the commonplace colours associated with the rainbow defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
- Whilst the visible spectrum and spectral colour are both determined by wavelength and frequency it is our eyes and brains that interpret these and create our perceptions after a lot of processing.
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