An emission spectrum is the electromagnetic radiation (EMR), such as visible light, a substance emits. Every element gives off a unique fingerprint of light, so analyzing the frequencies of this light helps identify the chemical that generated it. This procedure is called emission spectroscopy and is a very useful scientific tool. It is used in astronomy to study the elements present in stars and in chemical analysis.
Electromagnetic radiation can be described in terms of its wavelength — the distance between the crests of the waves — or its frequency — the number of crests that pass by in a given amount of time. The higher the energy of the radiation, the shorter its wavelength and the higher its frequency will be. Blue light, for example, has a higher energy and therefore a higher frequency and shorter wavelength than red light.
Types of Spectra
There are two types of emission spectrum. The continuous type contains many frequencies merging into one another with no gaps, while the line type contains only a few distinct frequencies. Hot objects produce a continuous spectrum, whereas gases can absorb energy then emit it at certain specific wavelengths, forming an emission line spectrum. Each chemical element has its own unique sequence of lines.
How a Continuous Spectrum is Produced
Relatively dense substances, when they get hot enough, emit light at all wavelengths. The atoms are relatively close together and as they gain energy, they move about more and bump against one another, resulting in a wide range of energies. The spectrum, therefore, consists of EMR at a very wide range of frequencies. The amounts of radiation at different frequencies vary with temperature. An iron nail heated in a flame will go from red to yellow to white as its temperature increases and it emits increasing amounts of radiation at shorter wavelengths.
A rainbow is an example of the continuous spectrum produced by the Sun. Water droplets act as prisms, splitting the Sun’s light into its various wavelengths.
The continuous spectrum is determined entirely by the temperature of an object and not by its composition. In fact, colors can be described in terms of temperature. In astronomy, the color of a star reveals its temperature, with blue stars being much hotter than red ones.
How Elements Produce Emission Line Spectra
A line spectrum is produced by gas or plasma, where the atoms are far enough apart not to influence one another directly. The electrons in an atom can exist at different energy levels. When all the electrons in an atom are at their lowest energy level, the atom is said to be in its ground state. As it absorbs energy, an electron may jump to a higher energy level. Sooner or later, however, the electron will return to its lowest level, and the atom to its ground state, emitting energy as electromagnetic radiation.
The energy of the EMR corresponds to the difference in energy between the electron’s higher and lower states. When an electron drops from a high to a low energy state, the size of the jump determines the frequency of the radiation emitted. Blue light, for example, indicates a larger drop in energy than red light.
Each element has its own arrangement of electrons and possible energy levels. When an electron absorbs radiation of a particular frequency, it will later emit radiation at the same frequency: the wavelength of the absorbed radiation determines the initial jump in energy level, and therefore the eventual jump back to the ground state. It follows from this that atoms of any given element can only emit radiation at certain specific wavelengths, forming a pattern unique to that element.
Observing Spectra
An instrument known as a spectroscope or spectrometer is used to observe emission spectra. It uses a prism or diffraction grating to split light, and sometimes other forms of EMR, into their different frequencies. This may give a continuous or line spectrum, depending on the source of the light.
A line emission spectrum appears as a series of colored lines against a dark background. By noting the positions of the lines, a spectroscopist can discover what elements are present in the source of the light. The emission spectrum of hydrogen, the simplest element, consists of a series of lines in the red, blue and violet ranges of visible light. Other elements often have more complex spectra.
Flame Tests
Some elements emit light mainly of just one color. In these cases, it is possible to identify the element in a sample by performing a flame test. This involves heating the sample in a flame, causing it to vaporize and emit radiation at its characteristic frequencies and give a clearly visible color to the flame. The element sodium, for example, gives a strong yellow color. Many elements can be easily identified in this way.
Molecular Spectra
Whole molecules can also produce emission spectra, which result from changes in the way they vibrate or rotate. These involve lower energies and tend to produce emissions in the infrared part of the spectrum. Astronomers have identified a variety of interesting molecules in space through infrared spectroscopy, and the technique is often used in organic chemistry.
Absorption Spectra
It is important to distinguish between emission and absorption spectra. In an absorption spectrum, some wavelengths of light are absorbed as they pass through a gas, forming a pattern of dark lines against a continuous background. Elements absorb the same wavelengths that they emit, so this can be used to identify them. For example, light from the Sun passing through the atmosphere of Venus produces an absorption spectrum that allows scientists to determine the composition of the planet’s atmosphere.