What is an emission spectrum? (with photos)

A radio telescope analyzes emissions from celestial bodies in the radio part of the electromagnetic spectrum.

An emission spectrum is the electromagnetic radiation (EMR), such as visible light, that a substance emits. Each element emits a unique fingerprint of light, so analyzing the frequencies of that 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.

The electrons in an atom can exist at different energy levels.

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 the wavelength and the higher the frequency. Blue light, for example, has a higher energy and therefore a higher frequency and shorter wavelength than red light.

Spectra types

Rainbows contain colors in the visible spectrum.

There are two types of emission spectrum. The continuous type contains many frequencies that merge with 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

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

Relatively dense substances, when hot enough, emit light at all wavelengths. The 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 EMR over a wide range of frequencies. The amounts of radiation at different frequencies vary with temperature. An iron nail heated in a flame goes from red to yellow to white as its temperature increases and emits increasing amounts of radiation at shorter wavelengths.

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A rainbow is an example of a continuous spectrum produced by the sun. Water droplets act like 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, 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 influence each other. 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 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 EMR energy corresponds to the energy difference between the upper and lower states of the electron. When an electron falls 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 certain frequency, it will subsequently 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 the atoms of any given element can only emit radiation at certain specific wavelengths, forming a pattern unique to that element.

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watching spectra

An instrument known as a spectroscope or spectrometer is used to observe the emission spectra. It uses a prism or diffraction grating to split light, and sometimes other forms of EMR, into its different frequencies. This can provide either a continuous or a linear spectrum, depending on the light source.

A line emission spectrum appears as a series of colored lines against a dark background. By looking at the positions of the lines, a spectroscopist can discover 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 tend to 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.

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