Monday, 29 March 2010

Colour is complicated - part 2

Part 1 is here.

I'm going to try to carry on from where I left off last time. Eventually I will get to primary colours and try to get my head around the colour yellow; it's a long journey though.


White light is a mixture of all the colours, as Sir Isaac Newton demonstrated with a prism. Different wavelengths bend differently at the interface of air and glass (or rain droplets) and this disperses the colours into the familiar rainbow spectrum. Blue light bends the most, red the least. The mnemonic “Richard Of York Goes Battling In Vain” is an aid to the recall of the colours of the rainbow - Red, Orange, Yellow, Green, Blue, Indigo, Violet. From the outside in!

Light continues at wavelengths beyond our vision. At longer wavelength than red is infrared which we feel as heat, but cannot see. Shorter then violet is high energy ultraviolet, which ages the skin and can trigger cancers - 'Slip-Slop-Slap' as the Australians say!

I don’t have an image, but Newton went on with a lens and another, inverted, prism to re-combine the colours and produce white again. Proof that white light contains all visible colours.

It is the colour dispersing nature of prisms which leads to chromatic aberration in lenses, and the complex optics needed to avoid it. But prisms, or their grown-up cousins diffraction gratings, can be used to split colours very finely to give almost single wavelengths, so-called monochromatic light, and this gives the fascinating science of spectroscopy.

So different colours are different wavelengths of light, and mixing all wavelengths gives white light, but what the hell is white paint?

It turns out that the energy carried by the visible wavelengths of light falls inside the region of the energies of electrons in atoms, and the vibrational energies of the chemical bonds between atoms. So if a bond is hit by just the right wavelength of light, it will absorb it and vibrate at that energy. A different wavelength will just pass through (transmit) or bounce off (reflect). The absorbed energy buzzes around for a bit till it's lost as heat and then the bond can absorb at that wavelength again.

A white surface reflects all visible wavelengths. It may well absorb wavelengths in the infrared or ultraviolet but we simply do not see that.


In white light a white painted wall is white. However, if you shine red light on it, it reflects red and looks red to the eye. Reflected blue light appears blue, green is green and yellow is yellow.

Now a white painted wall is not a mirror. Microscopic irregularities on the surface scatter the reflected light in all directions, when you have a perfectly smooth surface it reflects without scattering and the eye can reform an image. Glass can do this and, with a silvering behind it, we have the mirrors we all know. Frosted mirrors, or a frosted bathroom window, looks white because everything scatters.

So far we have been reflecting everything. What happens if the surface des not equally reflect all the wavelengths of light? What happens if some wavelengths are just absorbed, and vanish? Welcome to the real world of colour!

If I have a paint which absorbs red and blue light, and reflects green, it will look green to our eye.


Similarly, reflect blue and absorb everything else and you get blue. Reflect only red and you get red. Black absorbs every wavelength and reflects nothing.

If you want to make pink paint you mix white and a bit of red. The white part reflects everything, the bit of red still reflects red but absorbs a bit of everything else. The mix makes the shade.

I know this is all a big simplification because we haven't thought about the human eye yet, and the way we detect colour. I'll have a stab at that next time.

2 comments:

jill said...

Even though I majored in art and took courses in color, the entire concept is mind baffling to me. I just retained the basics!

PS...make sure to check out my post today! ;-)

Andy Holroyd said...

It is baffling Jill, but it may make more sense when I get around to how the eye works (or it may get more baffling).