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Spectroscopy

What is spectroscopy?

Spectroscopy is the study of how electromagnetic radiation interacts with matter - the  EM waves can be absorbed, emitted or scattered, depending on the type of matter which the radiation is interacting with (the matter can be atoms, molecules, ions, or solids), and the resulting spectra is a range of colours or lines. Spectroscopy is used by astronomers to obtain information on distant galaxies and stars such as what gases they are made up of (their chemical composition) and their distances (using their Doppler shift). 

Below is an outline of the 3 types of spectra, together with an explanation on what objects in the Universe would create them.

What’s a spectrum?

There are 3 different types of optical spectra (plural of spectrum) - continuous, emission line and absorption line.

Continuous Spectrum

A light source, such as a light bulb, emits visible light at a range of wavelengths, from violet to red, but our eyes see it as white light. If you pass this white light through a prism or diffraction grating however, it splits into a continuous spectrum of colours. 

This happens because as the light enters the prism, it is bent or refracted by differing amounts. The red wavelengths are refracted the least, whilst the violet ones are refracted the most. Then, as the light passes out of the prism into the air again, each wavelength of light is refracted once more, making the separate colours even more visible. This is shown in the diagram to the right. 

 

 

Image courtesy of Chris Palma/Penn State University

Absorption Spectrum

 

When white light is passed through a cool gas and then through a diffraction grating, a series of black lines cover the continuous spectrum of colours. This is because certain wavelengths of the light have been absorbed by the gas - this is an absorption spectrum, and can be seen in the image on the left.

 

In stars.... In a star, the central regions are very hot, so they emit white light. However, the light has to pass through the outer layers of the star, which are cooler, so some of the wavelengths are absorbed - this spectrum can then be analysed by astronomers to determine the gases in the star and even its temperature.

 

 

 Image courtesy of Chris Palma/Penn State University

Further out in the Universe... A similar thing happens when light from distant objects in the Universe reaches the Earth, as shown in the NASA image below. Light from a distant quasar passes through many clouds of gas as it travels to Earth, and with each cloud it passes through a 'fingerprint' dark line is made on the original spectrum of the quasar. These can be measured by special detectors either on Earth, or on the Hubble Space Telescope, and the composition and distances of the gas clouds can be found. 

 

Emission Spectrum
An emission spectrum is obtained when a hot gas emits light at specific wavelengths, as shown in the image on the right. If, for example, you had a sodium or neon lamp, and shone it through a diffraction grating, you would see a mainly dark spectrum, with a few bright lines on it. The position of these lines depend on the type of gas which is emitting the light, and the intensity of the lines depend upon the conditions of the gas, such as its pressure, density and temperature. These spectra are known as emission spectra, and can they help determine the elements in the gas.Below we have emission spectra of Hydrogen and Iron. 
 
 

Image courtesy of

How spectra are produced
Click on the options in the interactive animation below to investigate how the 3 different types of spectra are produced. 

 

Energy levels

The discrete lines in emission and absorption spectra arise because atoms of a given element can only emit or absorb energy at specific wavelengths. Energy is emitted when an electron in the atom loses energy and moves down from a higher energy level to a lower one. As a result of this, a photon of light is emitted which we see as a bright line of specific wavelength on an emission spectrum. The energy of this photon is equal to the difference in the two energy levels.

 

The emission spectra for Hydrogen and Iron are shown below:

 

Emission spectrum of Hydrogen
Emission spectrum of iron

 

To go up from one energy level to another in an atom, an electron must absorb a very specific amount of energy from a photon of light. When this happens a dark absorption line is seen in the spectrum

The absorption spectrum for hydrogen is shown below: 

The photon absorbed or emitted by electrons in an atom has an energy equal to the difference in the energy levels, given by:

                        hf=E1-E2                          (1)

where:

        h = Planck’s constant (6.63x10-34 Joule seconds, Js)
        f = frequency of photon (Hz)
        E1 = energy of level 1 (J)
        E2 = energy of level 2 (J)

Hydrogen Alpha

When an electron in a Hydrogen atom falls from the 3rd energy level to the 2nd energy level, it emits a photon of a specific wavelength and energy. The line which is visible on the emission spectrum is known as the Hydrogen Alpha or H-Alpha line.

Astronomers use this line to see how much hot ionized hydrogen gas is in gas clouds in the Universe.

It is important to remember that other lines also exist for hydrogen such as Hydrogen Beta, Hydrogen Gamma etc. where electrons fall from their energy levels down to the 2nd level, emitting photons of particular wavelengths and energy.

 

In a nutshell...
If a light source contains all possible wavelengths of light (i.e. if it's white light), then after passing it through a prism or diffraction grating, you will see a continuous spectrum.
An absorption spectrum is obtained when particular wavelengths of light are absorbed by a gas cloud between the source of light and the detector.

An emission spectrum is obtained when hot gas emits light of particular wavelengths.