The causes of colour

How we 'see' colour

The electromagnetic spectrum is made up of a range of wavelengths, energies and frequencies of light, but very little is actually 'seen' by the human eye. Most light is undetectable by our eyes, but the small portion of the spectrum that we do see, from 400-700 nanometres (10-9 metres), we perceive as the phenomenon of colour.

When white light, containing all wavelengths, strikes an object, some wavelengths are absorbed and others are reflected. When we look at an object, light emitted from the object is detected by the cones on the back surface of our eyes that are responsible for detecting and sending information about colour to our brain. Our brain interprets this information from the wavelengths of light that are reflected from the object; thus we can only see reflected light. Therefore, when we perceive an object as being red on the surface, the object is actually reflecting the wavelengths responsible for red light and absorbing all of the other colours of light. Objects that appear to be colourless or white are reflecting all wavelengths and objects that appear to be black are absorbing all wavelengths.

We use the colour wheel to explain further the relationship between absorbed and reflected light. Each colour has a complementary colour, which is shown as the opposite wedge in Figure 3. For objects that are coloured, the light we observe is the reflected light, whereas the complementary colour is the light that is strongly absorbed. For example, an object that appears red is reflecting red light (647-700 nm) and strongly absorbing its complementary colour, green (491-575 nm). Note that a colour wheel can be found in section 17 of the IB data booklet.

Coloured compounds

Biological pigments are compounds produced by metabolic processes that are capable of absorbing and reflecting light in the visible range of the spectrum, appearing coloured. Pigments are present in fruits, vegetables, blood and tissues and are responsible for the colours we see in living things. Dyes are synthetic colouring agents that can be added to foods, fabrics or other substances to provide colour. Pigments are produced by metabolic processes, whereas dyes are synthetic.

Whether or not a biological compound has the ability to absorb and reflect light in the visible range depends on the types of bonds present in the compound. Covalent bonds readily absorb light in the infrared range, causing the bonds to bend and stretch, making it possible to perform infrared spectroscopy. Electrons are also capable of absorbing light in the ultraviolet range, resulting in electrons being promoted to higher energy levels. Both infrared and ultraviolet absorption are outside our ability to sense these wavelengths of light, resulting in compounds that appear to be colourless.

Conjugation

Some compounds have a highly conjugated system; that is, a system of delocalised electrons from three or more overlapping p orbitals. When a compound has conjugation, there is greater stability in the molecule and the electrons are capable of absorbing light approaching the visible range. These compounds are known as chromophores. When there is extensive conjugation, the light absorbed has energies that correspond with the wavelengths in the visible range.

For example, the compound buta-1,3-diene is represented with a Lewis structure of:

However, it has been observed that the four electrons that make up the 'double' bonds are actually delocalised and the p orbitals for all four carbon atoms overlap, allowing the electrons to be spread out over all four carbon atoms. This makes for a more stable, conjugated system that is capable of absorbing higher energies of light.

The more conjugated the system is, the lower the energy, and the longer the wavelength that is absorbed, giving rise to different colours in the visible spectrum; however, the colours absorbed and reflected cannot easily be determined by the structure alone. The presence of functional groups can alter the ability of the compound to absorb light.

In order for the light reflected to be visible, there must be extensive conjugation – that is; more than about five conjugated double bonds. Many conjugated systems are linear. For example, beta-carotene, a compound of vitamin A, appears orange (585-647 nm), meaning that it is strongly absorbing light in the blue range (424-491 nm). Beta-carotene has a highly conjugated system and is therefore capable of absorbing light in the visible range.

Other conjugated systems consist of aromatic rings. For example, phenolphthalein is a chemical indicator often used in acid-base titrations to observe the end-point. The system of conjugation extends over the ring structures, resulting in a compound that is capable of absorbing light at 552 nm (green), therefore appearing pink-red in basic solution.