The classical way to make a spectrum - ala Isaac Newton - is to stick an optical prism into sunlight and twist it about until you get your rainbow. However, you might be disappointed the first time you try this, especially if you're thinking of the cover art of Pink Floyd's 'Dark Side of the Moon'! Here are some tips for you.
Prism
Light Source
The very best spectra are made with bright sunlight coming into a room. Its almost impossible to get a nice spectrum with just a lamp or torch, without at least a lens, and ideally also a slit.
If you don't have bright sunlight, or just want more control (e.g. for photography) then you'll need a bright, tight beam, e.g. from a slide projector. Regular ceiling lights won't work very well, because the light rays emanating from them arrive at the prism or grating at a wide range of angles, thus creating several overlapping spectra - which recombine to create white light!
The sharpest spectra come from a narrow beam of light - i.e. having the clearest colours. Consider two beams of white light parallel to each other, entering a prism, one above the other. Their spectra will overlap, and the colours recombine, to some extent. So if you have a single wide beam of light, its like having lots of narrow beams parallel to each other - all the spectra overlap and wash out the colour.
Slit
Make a slit 1 or 2 mm wide in cardboard or aluminium foil or whatever is handy, or stand a couple of things with straight edges 2 mm apart (i.e. use some ingenuity to make a narrow gap) and put it next to the lamp, and pass the light through that, onto the lens.
Take a piece of A4 card. Fold it in half (so the 2 short edges meet). Open it up to about a right-angle and stand it up on edge so the fold is vertical. Place the lens + holder next to the folded edge and mark the height of the centre of the lens, on the folded edge. Lay the card down and close it up, and use scissors to make a 1 mm cut, about 1 cm either side of the mark. Now cut parallel to the fold, so that a narrow rectangle of card falls out, and you are left with a narrow slit in the card.
Lens
Almost any 'magnifying glass' type of lens should do, i.e. convex both sides (or possibly flat one side, but not concave). If you have a magnifying lens (i.e. convex) you can try to create a parallel beam of white light by placing the light at the lens's 'focal point'. This is the spot where you can burn stuff if the sun was out; roughly the distance from the lens where an image of a distant lamp forms. Focal length is the distance from the lens to the focal point.
Another way to get the focal length is to use a big torch, or a table lamp, and focus its image on some kind of screen or wall. Arrange if you can for the lens to be about halfway between the lamp and its image. This may well take some fiddling about, but when you have the lamp and screen about equidistant from the lens, then the distance between them is the focal length!
Put the lens its focal length away from the slit, in a lens holder if you have one. Move it back & forth till you see a sharp image of the slit on the screen or wall.
Prism
Put your prism in the beam starting with one of the corners pointing to the lens, and rotate it till you see the spectrum on a screen placed nearby (could be a white wall). Hopefully you'll get a nice spectrum, but then you might be disappointed because its small.. just turn the screen if you can, so that the light from the prism hits it at an angle and so the spectrum will get stretched out.
Spectrum
The spectrum shape is rectangular - across the colours (red, orange, etc), and 'with' the colours (i.e. 90 degrees to the spectrum). Lets call the first dimension 'length', and the second one 'width'. The width of the spectrum depends on the length of the prism (assuming its lit all the way along, e.g. by sunlight) so a bigger prism, being longer, makes the spectrum wider. But not longer! This size depends only on how much the prism spreads out (disperses) the different wavelengths, and that depends on what the prism is made of (expensive glass generally being better than cheap plastic).
Diffraction Grating
For even better results, try using a diffraction grating instead of a prism. Diffraction grating (strictly speaking, diffraction grating replica) is a transparent medium on which there are thousands of closely spaced grooves. Each groove diffracts (i.e. deflects) light passing through it, by an amount that depends on the wavelength of the light. Different colours get deflected by different amounts, and so are spread out into the spectrum or rainbow. Since white light consists of all the colours of the rainbow, diffraction grating splits it up just as the prism does, only by a different method. The prism operates by 'refraction', which is the bending of light on travelling between different transparent media (such as air and glass) - again, by an amount that depends on the light's wavelength.
Put some diffraction grating right next to the lens. You should be able to get a good spectrum rather more easily. Diffraction grating is cool stuff! The groove in a CD makes a diffraction grating, so try reflecting your white beam (made by the lens) off the shiny side.
Place some diffraction grating on a mirror and hold it horizontally in the sunlight, and rotate it around its centre point. You should be able to reflect a brilliant spectrum onto the ceiling. See our rainbow mirror.
Similar methods of creating pretty rainbows (i.e. spectroscopes) enable scientists to analyse unknown substances in the laboratory, or to determine what distant stars are made of. If you make light by burning something (don't try this at home!) then that light has a characteristic spectrum, i.e. some colors are brighter than others. Why? Because electrons in the atoms become 'excited' by the heat, and jump up to a higher orbit; after a while they fall back down and emit a photon (a particle of light) whose energy (and hence, wavelength) depends on the difference between the 2 orbits.
Read more at: http://en.wikipedia.org/wiki/Rainbow "Rainbows are optical illusions and meteorological phenomena that cause a spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth's atmosphere. They take the form of a multicoloured arc, with red on the outer part of the arch and violet on the inner section of the arch. More rarely, a secondary rainbow is seen, which is a second, fainter arc, outside the primary arc, with colours in the opposite order, that is, with violet on the outside and red on the inside.
A rainbow spans a continuous spectrum of colours. Traditionally, however, the sequence is quantised. The most commonly cited and remembered sequence, in English, is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet. "Roy G. Biv" and "Richard Of York Gave Battle In Vain" are popular mnemonics.
Rainbows can be caused by other forms of water than rain, including mist, spray, dew, fog, and ice. Moreover, rainbows can have shapes other than a bow (arc), including stripes, circles, or even flames." (from Wikipedia)
Here is the wikipedia entry for 'rainbow'. Here is the wikipedia entry for 'optical spectrum'. Here is an extensive article explaining how rainbows form, from The University Corporation for Atmospheric Research.
