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Study Notes: Prisms and Separating White Light

White light is made up of a range of different coloured light but when we look at it we do not see the individual colours. What we see is the result of all the colours added together and this is white. A ray of white light is separated into its component colours which then re-combine to form white light again.

When white light is refracted through a prism as shown below the colours are separated from each other. Notice how the high frequency (or high energy) colours are refracted the most. The range of colours produced is known as the spectrum of visible light.

A ray of white light strikes a prism at an angle and emerges from the other side of the prism with the component colours separated.

White light is said to be polychromatic (meaning many wavelengths or colours). A prism can be used to obtain monochromatic (single colour) light using a slit – this is an opaque screen with a narrow opening that only allows a small amount of the spectrum to pass through.

By rotating the prism a different part of the colour spectrum shines on the slit opening, thereby allowing different colours to pass through the slit.

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The combination of a moveable prism and slit is called a monochromator. It is a simple device for producing light of only one colour within a spectrometric instrument. A monochromator using a prism works satisfactorily in the ultra-violet and visible regions and can also be used in the infrared region but works most efficiently at lower wavelengths.

Another type of monochromator uses a diffraction grating. These consist of reflecting metal or transmitting glass surfaces on which a large number of extremely fine parallel grooves have been cut. Diffraction gratings with about 13,000 grooves per cm are used for ultra-violet and visible light. Fewer grooves can be used for the infrared region.

Diffraction gratings could be called ‘super prisms’ because they separate wavelengths very efficiently and with high intensity.

The wavelength of the light reflected from the surface of a diffraction grating depends on the angle of incidence. At each angle, the incident and reflected beams cancel each other for all but a narrow range of wavelengths (1-5 nm bandwidth compared with 20 nm for a prism). Rotation of the diffraction grating changes the wavelength of the light that is reflected away from the surface.

Diffraction gratings produce light of far better optical purity than prisms or filters (filters act by absorbing some wavelengths and allowing a relatively broad range of wavelengths to be transmitted). A beam of white light strikes a diffraction grating and the reflected light is red in colour.


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