What’s an Emission Spectrum?

An emission spectrum is the electromagnetic radiation (EMR), such as visible light, that a substance emits. Each element emits a unique signature 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 analyses.

Electromagnetic radiation can be described in terms of its wavelength – the distance between wave crests – or its frequency – the number of crests that pass in a given period of time. The higher the energy of the radiation, the shorter its wavelength and the higher its frequency. For example, blue light 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 that blend into each other without gaps, while the line type contains only a few distinct frequencies. Hot objects produce a continuous spectrum, while gases can absorb energy and 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. Atoms are relatively close together, and as they gain energy, they move more and collide with each other, resulting in a wide range of energies. The spectrum, therefore, consists of EMRs 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 turn from red to yellow to white as its temperature increases and emits increasing amounts of shorter wavelength radiation.

A rainbow is an example of the continuous spectrum produced by the Sun. Water droplets act like prisms, dividing sunlight into its various wavelengths.
The continuous spectrum is determined entirely by the temperature of an object and not by its composition. Indeed, colors can be described in terms of temperature. In astronomy, a star’s color 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 that they do not directly affect each other. The electrons in an atom can exist at different energy levels. When all of an atom’s electrons are at their lowest energy level, the atom is said to be in its ground state. Because it absorbs energy, an electron can 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 in the form of electromagnetic radiation.
The energy of the EMR corresponds to the energy difference between the upper and lower states of the electron. When an electron transitions from a high-energy state to a low-energy state, the size of the jump determines the frequency of the emitted radiation. Blue light, for example, indicates a greater 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 subsequently emit radiation at the same frequency: the wavelength of the absorbed radiation determines the initial jump in the energy level, and therefore the eventual return to the fundamental state. It follows that the atoms of a given element can only emit radiation at certain specific wavelengths, forming a pattern unique to that element.
Spectra observation
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 can give a continuous or linear spectrum, depending on the light source.

A linear emission spectrum appears as a series of colored lines against a dark background. By looking at the positions of the lines, a spectroscopist can find out which elements are present in the light source. 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 test
Some elements emit light of mainly 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 emitting radiation at its characteristic frequencies and giving a clearly visible color to the flame. The element sodium, for example, imparts a deep yellow color. Many items can be easily identified this way.
Molecular spectra
Whole molecules can also produce emission spectra, which result from changes in the way they vibrate or spin. 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, certain 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 they emit, so this can be used to identify them. For example, sunlight passing through Venus’ atmosphere produces an absorption spectrum that allows scientists to determine the composition of the planet’s atmosphere.

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