Electromagnetic Spectrum

The electromagnetic spectrum is the decomposition of the electromagnetic Rayonnement according to its various components in term of Fréquence, energy of the Photon S or of Wavelength associated, the three sizes $\backslash \; nu$ (frequency), $E$ (energy) and $\backslash \; lambda$ (wavelength) being dependant two to two by the Constante of Planck $h$ and the Speed of light $c$, according to the formulas:

For the radio waves and the light, one uses usually the wavelength. Starting from x-rays, the wavelengths are seldom used: as one deals with very energy particles, energy corresponding to the Photon X or γ detected is more useful. This energy is expressed in electronvolt (eV), that is to say the energy of 1 electron accelerated by a potential of 1 Volt.

The white Lumière can break up into Arc-en-ciel using a prism or of a Diffraction pattern. Each “Couleur” corresponds to a wavelength; however, the Physiologie of the perception of the colors makes that a color seen necessarily does not correspond to a radiation single wavelength but can be a superposition of monochromatic radiations. The processes of decomposition of radiations in monochromatic waves are described in the article Spectrométrie .

The energy photons of visible light (purple) are to 3 eV. X-rays cover the range 100 eV with 100 keV. The rays γ are beyond 100 keV. Photons γ of more than 100 MeV (100  000  000  eV) emitted by a Quasar was detected.

History

The term spectrum was employed for the first time in 1666 by Isaac Newton to refer to the phenomenon by which a prism of glass can separate the colors contained in the light from the Sun.

Emission spectrum

See also: Emission spectrum

Excited atoms (for example by shocks) are de-energized by emitting a electromagnetic Onde. This one can break up into a superposition of sinusoidal waves (monochromatic) characterized by their wavelengths. The spectrum is consisted the whole wavelengths present. One materializes it using a prism of decomposition of the light in a whole of lines, the spectral lines, which correspond to different emitted the wavelengths (see example opposite).

The observation of the emission spectrum of the Hydrogène is done by means of a Geissler tube which comprise two electrode S and of hydrogen under weak Pression. The electrodes are subjected to a Potential difference of 1000 V. important the Electric field accelerates the Ion S present which by shocks excite the Atome S of Hydrogène. At the time of their de-energizing, they emit light which is analyzed by a Spectroscope. In all the cases one observes (in the visible one) the same spectrum made up of 4 lines (spectra of lines) to the wavelengths: 410 Nm, 434 Nm, 486 Nm, 656 Nm.

Niels Bohr will then interpret the emission of light by the emission of a Photon when the atom passes from an energy level to another. The emission spectrum of any element can be obtained by heating this element, then by analyzing the radiation emitted by the matter. This spectrum is characteristic of the element.

Absorption spectrum

See also: Absorption spectrum

The principle is exactly the same one as that of the emission spectrum: to an energy level given a wavelength corresponds. But instead of exciting matter (for example by heating it) so that it emits light, one lights it with white light (thus containing all the wavelengths) to see which wavelengths are absorbed. The energy levels being characteristic of each element, the absorption spectrum of an element is exactly complementary to emission spectrum. One makes use of it in particular in astrophysics: for example, to determine the composition of gas clouds, one studies their absorption spectrum while making use of stars being located in background like source of light. It is generally the goal of the spectrography of absorption: to identify unknown elements (or mixtures) by their spectrum.