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Regarding water-filled prisms, I have discovered that you can only get a
sufficiently dispersed spectrum with a 90-degrees prism. The refractive
index of water is significantly less than that of glass. So although a
60-degrees prism bends a narrow beam of white light nicely, it doesn't
spread out the colours enough to show them clearly.

That was why I recommended a rectangular fish tank. You can create a
spectrum in two different ways with this setup. You can bend a vertical
'slit' of light sideways through one corner of the tank onto a vertical wall
or screen. Or, as I suggested, you can bend a horizontal 'slit' of light
downwards, using the 'prism' formed by the water surface and the side of the
tank.

It really is effective, provided that you project a narrow, focused 'slit'
of very bright light. The incident light-beam must enter the water surface
at a low, grazing angle and the emerging beam shines downwards at a similar
angle, very close to the side of the tank.

Glass or Plexiglass tanks work equally well. 

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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