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Collection of electromagnetic waves that all have the same propagation speed c in space (c=299792.458 km/sec.) but are distinguishable through their wavelength λ (lambda). For a long time, the notion of light was reduced to so-called visible light (perceptible by the eye), i.e. a very narrow group of waves whose length varies between 0.3 micrometers (violet) and 0.7 micrometers (red). Our eyes are sensitive to these wavelengths because the sun emits its maximum amount of energy within this interval and they have adapted themselves to it over time. If our sun had been a cold star (red), we would certainly see the infrareds more and not the blue. However, all the electromagnetic waves have waves whose length varies from a millionth of a micrometer (gamma rays) to several kilometres (radio waves).

The colour of a body especially depends on the manner in which it reflects and absorbs the light that illuminates it. In the domain of the visible, an object that absorbs the short wavelengths (blue) and reflects the largest ones, appears red and vice-versa. And it is the chemical composition of bodies that mainly determines the absorption and distribution properties. The “colour” of a body therefore tells us about its composition and its identity. Of course, what is valid for so-called visible wavelengths of light is also valid for all electromagnetic waves (in this case, the detector is no longer the eye, but particular cameras or other detectors sensitive to precise wavelengths).  This is why, through misuse of language, we attribute a “colour” to each wavelength, for instance, and we speak of infrared or ultraviolet "colours" even though these waves do not provoke any coloured sensations in our eyes. Conversely, this is why we call a wave of a well-defined length a monochromatic wave (from khroma in Greek, which means colour).

Objective information on a source of light can only be obtained through its spectrum, i.e. all the elementary waves (monochromatic) that constitute an electromagnetic wave. Thus, the white light of the sun is constituted of a multitude of elementary waves, which are visible when a rainbow occurs or when the light is passed through a prism or a diffraction network. Little by little, it was discovered that each chemical element could be associated with a precise spectrum, different from the others (each body emits “light”; it simply needs to be heated or to produce an electrical spark within an enclosed space containing a gas under low pressure – emission spectrum). In the same way, atoms can also absorb light in spectral bands which are also characteristic (absorption spectrum). In other words, the spectrum of the light emitted or absorbed by a chemical element is its signature, a truly unique "barcode". This technique, called spectroscopy is a powerful means of analysis for recognising elements used today in all laboratories and industries.