Any object at any temperature is known to emit radiation (sometimes called thermal radiation), the characteristics of which depend on the temperature and properties of the object. At low temperatures, the wavelengths of the thermal radiation are mainly in the infrared region and hence are not seen by the eye. As the temperature of the object is increased, it starts to glow red after some time. At sufficiently high temperatures, the object appears white, as in the glow of a hot tungsten lightbulb filament. Careful study of thermal radiation shows that it consists of a continuous distribution of wavelengths from the infrared, the visible, and the ultraviolet portions of the spectrum.
The shape of this continuous distribution depends on the temperature and properties of the substance. In sharp contrast to this continuous distribution spectrum is the discrete line spectrum emitted by a low-pressure gas subject to electric discharge.
When the light from such a low-pressure gas discharge is examined with a spectroscope, it is found to consist of a few bright lines of pure color on a generally dark background. This is radically different from the continuous rainbow of colors seen when a glowing solid is viewed through a spectroscope. Furthermore, as can be seen from images of visible line spectra produced by emission in the visible range for elements such as hydrogen, mercury, and neon, the wavelengths contained in a given line spectrum are characteristic of the particular element emitting the light. The simplest line spectrum is observed for atomic hydrogen. Other atoms such as mercury and neon emit completely different line spectra. Every element emits a unique line spectrum which gives a practical and sensitive technique for identifying the elements present in unknown samples.
Another form of spectroscopy which is very useful in analyzing substances is absorption spectroscopy. An absorption spectrum is obtained by passing light from a continuous source through a gas or dilute solution of the element under analysis. The absorption spectrum consists of a series of dark lines superimposed on the otherwise continuous spectrum of that same element. In general , not all of the emission lines are present in the absorption spectrum.