I'm not giving a direct link to those pages, because that site is still developing and it is safer to give a link to the front page of the site. Note: If you are interested in understanding the relationship between colour absorbed and colour seen (beyond the very basic description above), find your way to lesson 2 ("Color and Vision") of "Light Waves and Vision" on The Physics Classroom. Copper(II) sulphate solution is pale blue (cyan) because it absorbs light in the red region of the spectrum. What this all means is that if a particular colour is absorbed from white light, what your eye detects by mixing up all the other wavelengths of light is its complementary colour. If you mix yellow and blue paint you don't get white paint. Mixing together two complementary colours of light will give you white light.īeware: That is NOT the same as mixing together paint colours. Blue and yellow are complementary colours red and cyan are complementary and so are green and magenta. An internet search will throw up many different versions!Ĭolours directly opposite each other on the colour wheel are said to be complementary colours. The diagram shows one possible version of this. If you arrange some colours in a circle, you get a "colour wheel". You can, however, sometimes get some estimate of the colour you would see using the idea of complementary colours. Mixing different wavelengths of light doesn't give you the same result as mixing paints or other pigments. Sometimes what you actually see is quite unexpected. You wouldn't have thought that all the other colours apart from some red would look cyan, for example. Working out what colour you will see isn't easy if you try to do it by imagining "mixing up" the remaining colours. The diagram gives an impression of what happens if you pass white light through copper(II) sulphate solution. We see this mixture of wavelengths as pale blue (cyan). The light which passes through the solution and out the other side will have all the colours in it except for the red. Copper(II) ions in solution absorb light in the red region of the spectrum. If white light (ordinary sunlight, for example) passes through copper(II) sulphate solution, some wavelengths in the light are absorbed by the solution. Why is copper(II) sulphate solution blue? Anyone choosing to use this spectrum as anything more than an illustration should be aware that it lacks any pretence of accuracy! The colours are only an approximation, and so are the wavelengths assigned to them. Important: This isn't a real spectrum - it's a made-up drawing. The diagram shows an approximation to the spectrum of visible light. Visible light has wavelengths from about 400 to 750 nm. Visible light is simply a small part of an electromagnetic spectrum most of which we can't see - gamma rays, X-rays, infra-red, radio waves and so on.Įach of these has a particular wavelength, ranging from 10 -16 metres for gamma rays to several hundred metres for radio waves. You will know, of course, that if you pass white light through a prism it splits into all the colours of the rainbow. Why do we see some compounds as being coloured? Be aware that this is only an introduction to what can grow into an extremely complicated topic. This page is going to take a simple look at the origin of colour in complex ions - in particular, why so many transition metal ions are coloured.
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