Spectrum

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Background

A spectrum can be produced in several ways. If sufficient apparatus is available, a ray box and equilateral prism can be supplied so that pupils can investigate the dispersion of a beam of light passing through a prism. However, it is often required that a large spectrum is displayed for class use. This can be done in several ways depending on the time, equipment and quality of spectrum required.
The question of how to produce a large "display" spectrum was posted a while ago on the Scitech-l discussion list. It is hoped that this will bring some of the ideas together

Quality of spectrum

Those who did "O" level physics using "Abbott" as their texts will know there was a lot of emphasis on a "pure" spectrum in the text. A pure spectrum in this sense meant that the colour bleeding from adjacent wavelengths was minimised. If you think of the beam as circles of light of different colours, then the colours/circles would have overlapped. This was a problem if you wanted to observe line spectra amongst other things.
Sometimes all that is required is a bright spectrum. Sometimes a "pure" spectrum is required. Sometimes particular areas of the spectrum are of interest, for example the Infra red or Ultra violet areas are wated to be investigated. In general the better the spectrum you require, the dimmer it will be and the more involved the projection. Spectra can be produced by two main methods: Prisms and diffraction gratings.

General description

White light is confined to a thin strip. The light from this strip is passed through either a prism or a diffraction grating and then either projected onto a screen, or viewed through a telescope.
The strip of light is focussed onto a screen using a lens, in the absence of prism or grating. When the prism or grating is introduced the spectrum will be in focus at the same distance from the lens but deviated by the dispersing agent.
For Prisms the light should come from below the base of the prism. Turning the prism to get an image of minimum deviation gives the best quality spectra. Pink Floyd fans will know the layout!

A ray diagram showing layout

The "purer" and larger the spectrum required the dimmer the spectrum will be.


Prism requirements

To produce a good spectrum a good quality, large, equilateral triangular prism is required of a highly dispersive material (e.g. flint glass). A large hollow prism can be filled with water or, for better results, Ethyl Cinnamate (-Chemists don't seem to have decided on a IUPAC name for this yet as different sources contradict each other!).
The use of a dataprojector and a spectrometer connected to a video camera allows the use of less expensive prisms.


Mounting arrangements

The prism may be mounted so that the triangular base is either vertical or, more usually, horizontal. The prism axis below refers to the prism's length- not the triangular section.

Horizontal mounting of prism (axis horizontal)

The prism is held at the ends so that the apex points toward the floor. Ideally the prism can be rotated about the axis slightly to allow the image to be directed. A clear "V" shaped trough can be used in this arrangement (if filled with the liquids mentioned earlier). The slit or light source should be horizontal. The light from this arrangement may be projected onto the ceiling or a screen.


Vertical mounting of prism (more usual)

The prism is placed on one triangular base, it should be easily turned either directly or by a turntable. The slit in this arrangement should be vertical. Projection should be onto a wall or screen.


Suggested methods

The following methods are suggested to produce spectra. They are listed in simplicity order.

  1. Glass lens bulb prism:Simplest method in terms of apparatus- Just requires a SBC (small bayonnet connector or "automotive") bulb, a lens of about +7Dioptres (15cm focal length), screen or wall and a prism. Adjust for minium deviation as descibed above in the general method. The line of the filament acts as a slit. Advantages: ease and brightness. Disadvantage spectrum isn't pure.This is very similar to the Nuffield experiment link to Practical physics
  2. Projector method:Very simple, A single slit (e.g.from the "nuffield grating set") is placed in to a slide projector. The light is focussed and a prism or diffraction grating placed in the path. Advantages: ease and purish spectrum. Disadvantages equipment.
  3. Bulb lens Slit:Effectively as with the Glass-lens-bulb method but the slit becomes the image. This can be used with diffraction gratings too, however light levels are low. A halogen bulb may be used but DUE TO UV this requires sheilding. Advantages: relatively easy and depending on slit width a relatively pure sspectrum. Disadvantage: can be small and dim.
  4. OHP diffraction grating:A thin gap of a few milimeters is made using the edge of aluminium foil at the centre of an OHP.The foil can be held down with card on top. A Diffraction grating of 600lines/mm or similar is placed in front of the head of the projector. Note the order of the colours is reversed to that of a prism. Focussing the slit before putting the grating in is advised. The "fringe colour" adjustment should be made if present on the front of the OHP for best "white".Note that an eye should be kept on the grating as certain OHPs will allow the gratiing to overheat. Advantages: easy to set up. Disadvantage slit has to be very small and the quality isn't to good, The colour order is reversed and this is not as a prism would show.
  5. Projector - Heat shield/ Slit:This allows an investigation of Hershell's (Infra-red) experiment. A Thermopile or infra red sensor should be mounted in card for this as a detector. A sacrificial slide needs to be made from glass and aluminium foil.This can be held in place with tape around the edges or glued. This should be made to the same size as a slide and a small, parallel gap is required . DO NOT USE the diffraction set as above as this will melt!. In the lens part of the projector there is an apparently clear glass sheet. Removing this allows IR to pass.The rest of the set-up is as earlier. Using white card holding the thermopile, the whole of the spectrum can be investigated. Advantages: Good alround method. Disadvantages: requites access to the projector's lenses, may melt slides if not replaced, only useful for a quick investigation (2 minutes).

External Links

[1] some investigations and demo ideas using the OHP method.

--D.B.Ferguson 21:38, 15 April 2007 (BST)
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