Wavelength

The physics of the nature of light is fascinating and for many years arguments raged between the scientists as to whether light behaves like waves of like particles. In other words, does it deliver its energy like a hail of bullets or like waves of water? Actually neither and both are true! This is called wave-particle duality. Part of the reason for science being wonderful is that it is not as cut and dried as many people like to think! However, it is possible to use a wave model for light and then the different wavelengths of light correspond to different colours.

Colour

The science of colour is amazing. However, it is not necessarily very straightforward. None of the theory of colour is on the primary curriculum but it produces such fun activities that it would be awful to miss them out!

If white light is passed through a prism (a triangular block of glass), then the light splits up into colours of the rainbow:

This is because white light is, in fact, a combination of these colours. The spectrum of the different colours or wavelengths of light has been produced on the screen.

Filters

If you pass white light through a red filter, then red light comes out the other side. This is because the red filter only allows red light through. The other colours (wavelengths) of the spectrum are absorbed. Similarly, a green filter only allows green light through. This is called colour by subtraction.

So when, for example, a red filter is placed in front of the spectrum above only the red part of the spectrum remains - the rest disappears.

N.B. The whole spectrum in NOT dyed red - the red part of the spectrum remains but the rest of the spectrum has been absorbed by the filter.

If a green and a red filter are placed together, what colours would you expect to be transmitted? (Answer - no colour will be transmitted, it will appear black. This is because a pure red filter only allows through red light and a pure green filter only allows through green light. Together this means that they allow no light and appear black.)

Similarly, any coloured surface, such as a jumper or a coat, is that colour as a result of colour by subtraction:

A red jumper is red when viewed in white light because all the wavelengths of the spectrum that fall on it are absorbed except red which is reflected into the eye. In practice, it is probably more accurate to say that the red jumper reflects more red light than other colours and so appears red. There are few pigments that are completely pure and only reflect one colour but some reflect mostly one colour.

A black coat appears black because it absorbs all the wavelengths of visual light that fall on it and no light is reflected into the eye from that object. It is because of this that black clothes become hotter on a sunny day that white ones.

It is interesting to consider what colour a blue dress will appear if viewed in a red light source! (Answer - black) Hence it is a good idea to choose clothes or carpet near a window or in daylight as they can appear rather different in shop lights. Paints are the result of colour by subtraction too. Red paint only reflects the red part of the spectrum, the rest is absorbed. Hence when many coloured paints are mixed together, the result tends to be murky and blackish! The different paints mixed together absorb just about all the light falling on them.

Contents

Sight, Safety, Light needs a source, Speed of Light