When discussing light, a spectrum is a linear scale of spectral colours organised by wavelength.
- A spectrum or spectra (plural) are produced naturally when light is refracted by a prism or by rain.
- A diagram of spectral colours is usually presented in the form of a spectrum, a linear band organised by wavelength with red at one end and violet at the other.
- Spectra are often shown in an elongated form so that as many different gradations of colour can be seen as possible.