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To create a spectrum with a prism, you first need a thin band of parallel
light rays in the form you get when sunlight shines through a narrow slit.
You can easily create this effect with a good old 35mm slide projector and a
special 'slide' made from a 50mm square piece of card with a 1mm wide slit.

If you shine a wide beam of light through a prism, you just see a patch
white light with faintly colour-tinged edges.

The easiest way to achieve the effect you describe is to put enough water in
a small glass aquarium tank and mount that above head-height. Then shine
your projector beam almost horizontally but slightly downwards into the
water, with its 'slit' arranged horizontally. Adjust the focus and you'll
see a really beautiful spectrum on the floor below the tank.

My feeling is that this is more likely to produce the effect you want than a
diffraction grating.  

Promoting public engagement with science
through a contagious delight in phenomena
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Give people facts and you feed their minds for an hour.
Awaken curiosity and they feed their own minds for a lifetime.
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Ian Russell

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