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Spectral Lines

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Spectral lines carry a wealth of information concerning the medium in which they form. The line shape and intensity are influenced by the abundance of the element producing the line, by the temperature and density structure of the atmosphere and by the state of motion and magnetic field strength in the atmosphere.


Spectrum Analysis (emission spectrochemical analysis) or also known as spectral analysis is the process of investigation of the distribution of energy emitted by a radiant source, such as visible light, arranged in order of wavelengths. The information given by spectral analysis is used in chemistry and is also a major tool for discoveries in fields like astrophysics and atomic theory.

What are Spectral Lines?

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Spectral Lines Definition

William Hyde Wollaston and Josef Von Fraunhofer used a method to examine sunlight. They found that the normal spectrum was crossed by dark lines. These line represent wavelengths where there is less radiation than at nearby wavelengths. The line like appearance comes from the fact that at each wavelength we are seeing the image of the slit. It is this line like appearance that leads us to call these features spectral lines.

The spectral lines in absorption as well as emission spectroscopy are not exactly sharp lines but appear as broad peaks. The width of spectral lines depends on the natural line width of an atomic or molecular transition and resolving power of the instrument. The resolving power of the instrument can be improved by better design and use of more sophisticated components but the natural line width arises due to the fact that the energy levels in atoms and molecules are not precisely determined due to contributing factors

Absorption Lines

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An absorption spectrum results when radiation passes through a cool gas. When a beam of white light was passed through a tube containing some gas the spectrum showed dark lines. The gas was absorbing energy in a few specific narrow wavelength bands. we refer to the lines as absorption lines.

By the nature of quantum mechanical change induced in the molecule absorption lines are classified. The rotational lines occur which are found in microwave spectral region When the molecule rotational state is changed. When vibrational state of a molecule is changed then vibrational lines occur which are found in infrared region. When electronic state of a molecule is changed then electronic lines occur which are found in visible and ultra violet region.

Spectral Lines of Elements

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The spectral lines of chemical elements are passports to their identity. From the observation of the particular bands of bright lines in a spectroscope it is possible to identify the chemical element which produces it. Such bright lines are called emission lines.

The spectral lines of different elements are shown below. The third line represents the neon spectral lines.

Spectral Lines of Elements

Helium Spectral Lines

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Helium was first identified as a new element by its fluorescence spectrum in the solar corona. When viewed directly the discharge helium lamp appears to emit pinkish white light. When viewed through the diffraction grating however each wavelength in the light is diffracted at a different angle.

Helium Spectral Lines

The different wavelengths are of course only few of the discrete components in the fluorescence spectrum of the helium atoms. In the helium discharge tube a large number of neutral helium atoms are present along with a small number of free electrons and a matching number of ionized helium atoms to conduct electrical current.

Hydrogen Spectral Lines

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The spectral lines of hydrogen correspond to particular jumps of the electron between energy levels. The spectral lines are grouped into series. Lines are named sequentially from the longest wavelength means lowest frequency of the series.

Hydrogen Spectral Lines

There are other series in the hydrogen spectrum which are invisible to our naked eye. Their presence is recognized by recording them on photographic plate. They are named as per the range of the wavelengths of the lines appearing in those series as well as on the basis of the names of the scientists who discovered those series.

An important point to be noted is that the wave numbers and wavelengths of all the lines in all series can be calculated by using the simple equation quoted above.

Atomic Spectral Lines

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Atomic spectra are of two types
  1. Atomic absorption spectra
  2. Atomic emission spectra
When an absorbing substance is introduced between a source emitting continuous spectrum and a spectroscope it is found that some regions of the continuous spectrum are missing. This is due to absorption of light. Generally absorption occurs whenever light passes through any transparent substance. In addition, certain substances strongly absorb light corresponding to certain parts of the spectrum which is called selective absorption.

Atomic Spectral Lines

Again this atomic emission and absorption spectrum are classified as line spectrum, band spectrum and continuous spectrum.