Visible light is just the part of the ‘electromagnetic spectrum' (see Appendix 1) that is detected by the human eye. Visible light is a mixture of colours and can be split into different colours by a prism or rainbow. These colours consist of waves of different wavelengths and a typical human eye can see light with wavelengths from about 400 to 700 nm:

Drag the slider across the spectrum to see the corresponding wavelength and energy of the visible light.

Objects around us have colours because they absorb these wavelengths to different extents. Visible light interacts with the electrons in molecules. When light is absorbed by a molecule, it can have two consequences for the electrons:

• Electrons can become briefly excited before quickly returning to normal and emitting the light
• Electrons can become so excited that they cause bonds to break.

The first of these processes is non-destructive and is the origin of the colour of dyes and other solutions and solids. The second process causes chemical reactions to occur which may be beneficial (such as photosynthesis in plants) or unwanted (such as sunburn). Chemicals that interact with light in this way are called photochemicals.

Origin of Colour

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White light, such as sunlight, is a combination of all colours. Materials that are black absorb all the visible light which is shone on them. Materials that are white or colourless absorb no visible light.

The colours of other objects are due to the particular wavelengths of light that they absorb. A solid that absorbs red light and reflects all the other colours will appear green. We perceive a green colour when we see white light with the red part removed. Similarly, red wine absorbs green light and allows the other colours to pass through. White light with the green part removed is perceived by us as a red colour.

Red and green are examples of complementary colours . Complementary pairs include:

• Red and green
• Blue and orange
• Yellow and purple

Test your perception of complementary colours.

Complementary colours have been known to artists for thousands of years. In Newton 's colour wheel, complementary colours are on opposite sides of the wheel.

Drag the slider across the spectrum or click on one of the coloured segments to pick the absorbed colour and to show the complementary colour according to the colour wheel.