Principle of operation of a spectroscope

The spectroscope is an optical system intended to observe the luminous spectra. Spectroscopy principles
Spectroscopy principles

We use the source to light a narrow slit. A first collimating lens is placed on the path of light to send a parallel beam on a prism or a grating. After the dispersion of light, a second lens projects on a screen the image of the slit. Each line correspond to a wavelength. This serie of lines, constitutes the spectrum of the light source.

Example:

  • White light is broken up into rainbow like spectral lines from red to blue (visible light), it is a continuous radiation spectrum or continuum. A particular type of continuous spectrum is that emitted by the black body.
  • A incandescent gas gives bright lines of specific wavelengths, it is an emission spectrum and the position of the lines are characteristic of this gas.
  • The same cold gas is between the source (white light) and the spectroscope. It absorbs some of the radiations emitted by this source. Dark lines are observed at the same position than the bright lines of the previous spectrum. It is an absorption spectrum.

    Very quickly the scientists understood that this was a great tool for analyzing the composition of stars.

    The chemical composition of a star can be determined thanks to the absorption lines in the stellar spectra. Each chemical element is responsible for a set of absorption lines and its wavelength can be precisely measured in a laboratory environment. It is then possible to match those precise measures with the recorded sellar spectrum and to determine the chemical composition of the star.

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