Light of a single wavelength is known as monochromatic light. The term light signifies the visible and near-visible portions of the electromagnetic radiation. These radiations exist over a huge range of wavelengths, ranging from picometers to kilometres. This range of wavelengths is collectively referred to as the electromagnetic spectrum. To obtain light of specific wavelength, one must use a form of light filter known as a monochromator to filter the unwanted wavelengths. Monochromators are generally variable so that different wavelengths can be selected at different times for different applications. Different chemicals will absorb or emit specific wavelengths of light, so this can be a diagnostic tool for chemical identification.