Emission Spectra Absorption Spectra and Continuous Spectra

Main Difference – Continuous Spectrum vs Line Spectrum

A spectrum is a set of wavelengths that is characteristic of electromagnetic radiation which is emitted or absorbed by a particular object, substance, atom or a molecule. The colours of the rainbow, microwaves, ultraviolet radiation and x-ray are some examples. A spectrum is characteristic of the elements present in the material considered. Continuous spectrum and line spectrum are two types of spectra; their main difference is that continuous spectrum contains no gaps whereas line spectrum contains many gaps. However, it is important to know more about absorption spectrum and emission spectrum, which are the two main spectra, before learning the difference between continuous spectrum and line spectrum since both absorption and emission spectra are responsible for creating continuous and line spectra.

This article explores,

1. What is an Absorption Spectrum
2. What is an Emission Spectrum
3. What is a Continuous Spectrum
4. What is a Line Spectrum
5. What is the difference between Continuous Spectrum and Line Spectrum

What is an Absorption Spectrum

When electromagnetic radiation is passed through a certain material, some characteristic wavelengths are absorbed by the elements in the material. However, the re-emitted photons are not re-emitted in the same direction. Due to the absence of this absorbed electromagnetic radiation, dark lines appear in the spectrum. An absorption spectrum is plotted with absorbance in the y-axis and wavelength or frequency in the x-axis. Absorption spectra are used in various analysis techniques such as atomic absorption spectroscopy and UV-absorption spectroscopy. These techniques are used in identifying a certain species in a given mixture or confirming the identity of a particular species.

What is an Emission Spectrum

When a beam of electromagnetic radiation is sent through a sample of atoms or molecules, the electrons in them absorb energy and transfer themselves to a higher energy state. Then they fall back to the earlier energy states they occupied by giving away additional energy they absorbed. When the released energy is plotted against the wavelength it is called an emission spectrum.

An absorption spectrum is denoted by the dark lines in a bright background whereas the opposite is shown in an emission spectrum. These two are the reverse of each other. For a given element, the absorption lines correspond to the frequencies of the emission lines. This is because the energy absorbed by electrons of a certain element to reach higher energy levels are emitted when they return to the previously occupied energy level.

What is a Continuous Spectrum

A continuous spectrum is created by putting both absorption and emission spectra together. The main requirement for a spectrum to be a continuous spectrum is that it should contain all the wavelengths within a given range. Visible light, when diffracted, produce a continuous spectrum. The rainbow contains seven colours which fade into each other without leaving any gap. When a black object is heated to glow it emits radiation in a continuous spectrum.

However, scientists say that continuous spectrum also contains gaps and can be seen only when analysed by a spectrometer. An ideal continuous spectrum should not contain and gap whatsoever. This could only be achieved at perfect lab settings and is very rare.

Figure 1: Formation of Continuous Spectrum

What is a Line Spectrum

Line spectra are generated only in either absorption spectrum or emission spectrum. It shows separate isolated lines in a given spectrum. These can be absorption lines which appear as dark lines in a bright background or bright emission lines that appear on a dark background.

Line spectra can be produced using the same source of light which produces a continuous spectrum. Under high pressure, a gas produces a continuous spectrum. However, under low pressure, the same gas can give rise to either an absorption or an emission spectrum. If the gas is cold it gives rise to an absorption spectra. If the gas is produced in association with heat, it produces an emission spectrum.

Figure 2: Emission spectrum of iron

Difference Between Continuous Spectrum and Line Spectrum

Definition

Continuous Spectrum: Continuous spectrum is the superimposed image of both absorption and emission spectra.

Line Spectrum:Line spectrum is either an absorption spectrum (dark lines in a bright background) or an emission spectrum (bright lines in the dark background).

Gaps

Continuous Spectrum: Continuous spectra contain no observable gaps.

Line Spectrum: There are huge gaps between lines.

Wavelength

Continuous Spectrum: Continous spectrum contains all the wavelengths of a given range.

Line Spectrum: Line spectrum contains only a few wavelengths.

Examples

Continuous Spectrum: Rainbow and black body radiation are examples of a continuous spectrum.

Line Spectrum: Emission spectra of hydrogen and absorption spectra of hydrogen are examples of line spectrum.

Conclusion

The main difference between continuous spectrum and line spectrum is that line spectra can be seen as either isolated emission lines or absorption lines, with huge gaps between them, whereas continuous spectra do not contain gaps and can be produced by superimposing the emission and absorption spectra of the same element.

Reference:
1. Helmenstine, Anne Marie. "Spectrum Definition." About.com Education. N.p., 07 Aug. 2016. Web. 21 Feb. 2017.
2. "Difference between continuous and line spectrum of hydrogen." Chemistry Stack Exchange. N.p., n.d. Web. 21 Feb. 2017.
3. "Atomic theory: 1.32 – Continuous and line spectra." IB Chemistry Web. International Baccalaureate Organization, n.d. Web. 21 Feb. 2017.
4. Darling, David. "Absorption Spectrum." David Darling.Com. N.p., n.d. Web. 21 Feb. 2017.
5. Volland, Walt. "Emission Spectroscopy: Element identification.". N.p., 31 Mar. 2015. Web. 21 Feb. 2017.
6. Barnes, Joshua E. "Spectra in the Lab." Institute for Astronomy. University of Hawaii, n.d. Web. 21 Feb. 2017.
7. "What is Continuous Spectrum?" Continuous Spectrum. N.p., n.d. Web. 21 Feb. 2017.
8. "Emission and absorption spectra." SIYAVULA. N.p., n.d. Web. 21 Feb. 2017.

Image Courtesy:
1. "Spectral lines en" By User: Jhausauer – Author (Public Domain) via Commons Wikimedia
2. "Emission spectrum-Fe" By User: nilda – Own work (Public Domain) via Commons Wikimedia

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