Light is made up from wavelengths, which correspond to the colour of light. The wavelengths that make up visible light are part of the electromagnetic spectrum, which includes other forms of energy such as radio waves, x-rays, and microwaves.
The light spectrum is the range of wavelengths that make up visible light. At each of these wavelengths a particular colour of light is produced, and the whole spectrum combined creates white light.
The effect of splitting white light into its different wavelengths, or colours can be seen when light is shone through a prism, or in the colours produced by a rainbow.
At one end of the light spectrum is ultraviolet (UV) light, with a short wavelength of between 300 and 350 nanometres (nm), as the wavelengths increase the spectrum passes through violet, indigo, blue, green, yellow, orange, and red before entering infrared light at around 700-750nm.
A shorter wavelength indicates a better ability to pass through water, and a longer wavelength is more likely to be absorbed and lost.
In the aquarium, the spectrum of light produced by a light source (such as fluorescent tubes) affects the intensity of the colours we see in the aquarium, and also the amount of light which plants can use to photosynthesise.
Most standard aquarium fluorescent tubes peak in the red and blue areas of the spectrum, producing a tube that appears to glow a purple colour. As the light passes through the water, the red light (long wavelength of 650-675nm) tends to get quickly absorbed and lost. The blue light (short wavelength of 400-500nm) passes through easily and reflects of objects in the aquarium so that we can see them.
Because different parts of the light spectrum pass through the water at different rates, a light tube that appears purple in colour does not produce a purple appearance in the aquarium. The best tubes for showing up the colours of fish and providing an appearance that is pleasing to the eye are 'full spectrum tubes' which provide an intense blue-white light. Full spectrum tubes can be used together with tubes designed for aquarium plants to disguise the yellowish hue they may give off.