Understanding Spectra

The SCISAT-1 ACE (Atmospheric Chemistry Experiment) mission will collect solar spectra. In general the spectra collected will be modified solar spectra because the ACE mission will collect the spectra of the sunlight which have been transmitted through the Earth's atmosphere. The passage of sunlight through the Earth's atmosphere modifies its spectrum. These changes can be related to the physical and chemical properties of the air through which the sunlight has passed.

To understand the process we must first understand something about the spectra themselves.

Three types of Optical Spectra

hree types of Optical Spectra
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When ordinary light is passed through a prism (or diffraction grating) a spectrum is produced owing to the fact that the incident light is really the "blended" effect of many different wavelengths (colours).

Spectra can be classified into three general categories.

  1. Continuous (top)
  2. Absorption (centre)
  3. Emission (bottom)

Continuum Spectrum

Continuum Spectrum
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Incandescent sources, such as light-bulb filaments and the surface of the Sun, yield a spectrum which covers all wavelengths. The strongest colour depends only upon the temperature of the source.

Such a spectrum is illustrated in the diagram shown above.

If the spectrum were to be recorded on a spectrograph, the chart would look similar to the one drawn below the spectrum. The wavelength (colour) at which the intensity is greatest, is determined by the temperature of the source.

By measuring the wavelength of peak intensity, it is possible to make accurate temperature measurements of any hot object, ranging from molten metals (in a blast furnace) to the surface temperature of distant stars.

Absorption (Line) Spectrum

Absorption (Line) Spectrum
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When a continuous spectrum is observed after its light has passed through a gas, the observed spectrum appears modified. The spectrum is observed to contain "dark" lines ... some colours appear to be missing. This is the type of spectra collected by the SCISAT-1 ACE (Atmospheric Chemistry Experiment) mission.

This effect is seen in sunlight after it has passed through the outer layers of the Sun's atmosphere and also after passing through the Earth's atmosphere.

It is now known that each type of atom absorbs light of a specific wavelength (or combination of wavelengths), the absorbed wavelength being a unique characteristic of each type of atom.

Determination of the absorbed wavelengths can be used to identify the chemical composition of the gas through which the light has passed.

By comparing the Sun's spectrum above the Earth's atmosphere to it's spectrum after passing through the Earth's atmosphere, it is possible to determine the composition of the gases through which the light has passed.

Emission (line) Spectrum

Emission (line) Spectrum
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In the late 19th century scientists noticed that when elements were ionized by passing a high voltage through their low pressure vapour (in sealed glass tubes similar to those which are used for neon signs), they would produce a spectrum composed of a series of bright lines. They also noticed that the emitted colours of the lines and their spacing were unique to every element. These emission spectra could be used to identify any substance which could be ionized.

In fact, it was discovered that the emission lines of each element where identical to the lines that the same elements produced in an absorption spectrum.

This astounding discovery set off a rush among scientists around the world to identify the emission lines of practically every (and any) substance that they could get their hands on. Huge catalogues were compiled listing the emission (an hence absorption) wavelengths of thousands of compounds and their elements of which they are composed.


By matching up the laboratory generated catalogue of emission lines with the FraunhoferFootnote 1 lines observed in absorption spectra produced by the atmosphere one can figure out the chemical nature of the material causing the absorption lines.


Footnote 1

Joseph Fraunhofer (1787-1826): A talented optician, he was able to produce very good solar spectra which lead to the study of the lines which now bear his name.

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