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Absorption Spectra - Light .. Physics

Absorption Spectra

The light-emitting material is characterized by its ability to absorb all light of the same frequency. If the material falls on the path of a light beam or ultraviolet rays, its atoms absorb all the photons resulting from the allowed electrons' bounce. However, the intensity of this absorbed light is related to the type and thickness of the material on the one hand, and to the frequency of the transmitted rays on the other hand. In general, all materials can, to some extent, absorb a portion of the visible spectrum and the ultraviolet spectrum.

The spectrometer is a device for recording both absorption and emission spectra. The rays enter it through a narrow slit, and fall on a quartz prism that divides them into components of different frequencies, all of which are concentrated at focal points on the photographic plate. If the spectrum of a light source is a continuous spectrum with constant intensity, then its absorption spectrum resulting from the passage of its rays through a gaseous medium is composed of dark lines corresponding to the frequencies absorbed by the atoms of the medium itself. These lines can be combined into bands as in emission spectra and for the same reasons.

(Above) The apparent wavelength of light is determined by the observer's movement relative to the source.

(Above) Fraunhofer lines in the solar spectrum.

(Bottom) Comparison between the emission spectrum and the absorption spectrum.

In 1814, von Fraunhofer noticed that the solar continuous spectrum is lined with hundreds of dark lines called Fraunhofer lines, indicating the presence of some elements, such as hydrogen and helium, in the sun's atmosphere, since the continuous spectrum emanates from the center of the sun itself, and its atmosphere plays the role of the medium absorbing rays.

It is known that each star has its own absorption spectrum that distinguishes it from other stars, and gives us abundant information about its speed whether it is heading towards the Earth or away from it. According to the Doppler effect, the spectrum of a moving star is usually shifted towards the red end of the visible spectrum or towards its blue end.