Physics > Waves > The science of colour

Colours are produced by 15 different physical and chemical mechanisms, that are described in the book by Nassau K. called Color for Science, Art and Technology.

Colour is actually light; white light is composed of all other colours put together. This was proved by Newton centuries ago, when he split white light with a prism and then put them back together using a second prism, inverted with respect to the first. This is mechanism of colour production is called dispersion. Let´s look at others. Colours can be separated because they have different frequencies, increasing from red to violet.

Most of these 15 mechanisms consist of electrons moving between different orbitals, either in atoms and molecules. Organic molecules need long conjugated structures to interact with visible light, whereas compounds containing metals can do it in a much smaller size. These topics are usually studied in chemistry. In fact, when people think of colours the idea of dye or pigment comes to mind; those materials are produced by the chemical industry (in fact they marked the beginnings of the chemical industry). So to a lot of people colour is caused by chemical mechanisms. I will describe here the mechanisms studied in physics: I will focus on the colour mechanisms related to geometrical optics and also heat.

The red colour produced by incandescence, for instance in the case of the heating of a metal (which gets red hot) requires a hight temperature, differently from the red of flowers, for instance, that don´t need that. It also happens on those light bulbs with a metal filament on the middle, in limelight (heated calcium carbonate), flames (where tiny pieces of carbon glow), etc..

The rest of the causes for colour I will mention here are a result of our current chapter, geometrical optics.

Dispersion,as mentioned inn the introduction, causes colour by splitting white light. The cause is actually refraction; dispersion is just the name given for the splitting of the colours because different frequencies have different refractive indexes, and as a result refract at a different angle. The angles increase in going from red to violet as we are used to see in prisms and rainbows, where the light is diffracted by drops of water.

The production of colour by interference I have explained here.

Finally. colour by scattering is produced when light scatters in tiny particles. The typical example is the colour of the sky. Blue light because of its smaller wavelength is capable of interacting with gases and tiny particles. e. As a result the sun becomes yellow, because its white light was emptied of the blue component. At sunset or sunrise, when the sun is low in the sky, there are larger particles on the path of its light rays so more colours get scattered, even the red!

This can be demonstrated easily using water with milk diluted to acts as the dust (check this experiment in here)

So , to summarise, last make a list of the physical causes of colour:

Heat

1) Colour by incandescence

Geometrical optics

2) Colour by dispersion (refraction)

3) Colour by interference (from diffraction or thin-film interference)

4) Colour by scattering