# The study of light emissions and absorptions

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The study of light emissions and absorptions
Spectroscopy The study of light emissions and absorptions The scientific study of objects based on the spectrum of the light they emit is called spectroscopy.

The Visible Spectrum Also known as a continuous spectrum

Newton’s Colour Wheel Newton divided the spectrum into seven named colors: red, orange, yellow, green, blue, indigo, and violet; or ROY G. BIV. He chose seven colors out of a belief, derived from the ancient Greek philosophers, that there was a connection between the colors, the musical notes, the known objects in the solar system, and the days of the week.[1][2] The human eye is relatively insensitive to indigo's frequencies, and some otherwise well-sighted people cannot distinguish indigo from blue and violet. For this reason some commentators including Isaac Asimov have suggested that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet.

Spectral Colours Although the spectrum is continuous and therefore there are no clear boundaries between one colour and the next, the following ranges may be used as an approximation. The unit nm refers to nanometers: 10-9 m

Dispersion of White Light
Newton first used the word spectrum (Latin for "appearance" or "apparition") in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different coloured bands. Newton hypothesized that light was made up of "corpuscles" (particles) of different colours, and that the different colours of light moved at different speeds in transparent matter, with red light moving more quickly in glass than violet light. The result is that red light was bent (refracted) less sharply than violet light as it passed through the prism, creating a spectrum of colours.

A Rainbow A rainbow is an optical and meteorological phenomenon that causes a nearly continuous spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth's atmosphere. It takes the form of a multicoloured arc, with red on the outside and violet on the inside. More rarely, a double rainbow is seen, which includes a second, fainter arc with colours in the opposite order, that is, with violet on the outside and red on the inside. Even though a rainbow spans a continuous spectrum of colours, traditionally the full sequence of colours is most commonly cited and remembered as red, orange, yellow, green, blue, indigo and violet. ("Roy G. Biv" is a popular mnemonic.)

Creating a Spectrum A broad spectrum of emissions from a non-specific light source - eg. Household light globe

Atomic Emission Lines An emission line is formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy state, emitting a photon of a particular energy and wavelength. A spectrum of many such photons will show an emission spike at the wavelength associated with these photons.

Atomic Emission Principles
When the electrons in the element are excited, they jump to higher energy levels. As the electrons fall back down, and leave the excited state, energy is re-emitted, the wavelength of which refers to the discrete lines of the emission spectrum. An element's emission spectrum is the relative intensity of electromagnetic radiation of each frequency it emits when it is heated (or more generally when it is excited). When the electrons in the element are excited, they jump to higher energy levels. As the electrons fall back down, and leave the excited state, energy is re-emitted, the wavelength of which refers to the discrete lines of the emission spectrum. Note however that the emission extends over a range of frequencies, an effect called spectral line broadening.The term often refers to the visible light emission spectrum, although it extends to the whole electromagnetic spectrum, from the low energy radio waves up to high energy gamma rays. The emission spectrum can be used to determine the composition of a material, since it is different for each element of the periodic table. One example is identifying the composition of stars by analysing the received light.

Electron Excitation The Hydrogen Atom
The Bohr model of the hydrogen atom, where negatively charged electrons confined to atomic shells encircle a small positively charged atomic nucleus, and that an electron jump between orbits must be accompanied by an emitted or absorbed amount of electromagnetic energy hν. For every possible transition back to ‘ground state’, there exists a line in the emission spectrum corresponding to a specific frequency of emitted energy. These frequencies don’t have to fall in the visible region of the spectrum.

Hydrogen Tube and Emission Bands

Emissions from an elemental gas
Emissions from a single element in the gas discharge tube.

Emission Spectrum - H

Emission Spectrum - He

Emission Spectrum - Na

Emission Spectrum - Fe

Emission Lines from a number of elemental sources

Fireworks Red - Strontium and Lithium salts Orange - Calcium salts
Yellow - Sodium salts

Fireworks Green - Barium salts Blue - Copper salts
Gold - incandescence of iron and charcoal White - white hot metals

Spectral Lamps Sodium Lamp Potassium Lamp Cadmium Lamp

Spectral Lamps Helium Lamp Thallium Lamp Neon Lamp

Spectral Lamps Zinc Lamp Rubidium Lamp Mercury Sulfide Lamp

Atomic emissions beyond the visible spectrum
Note that the emission extends over a range of frequencies. The term often refers to the visible light emission spectrum, although it extends to the whole electromagnetic spectrum, from the low energy radio waves up to high energy gamma rays.

Atomic Absorption Lines
An absorption line is formed when an electron makes a transition from a lower to a higher discrete energy state, with a photon being absorbed in the process. These absorbed photons generally come from background continuum radiation and a spectrum will show a drop in the continuum radiation at the wavelength associated with the absorption.

Comparing different spectra

Astronomical Spectroscopy
Involves the scientific exploration and analysis of the properties of distant objects Involves the observation of spectra at very high spectral resolutions Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects. Spectroscopy can be used to derive many properties of distant stars and galaxies, such as their chemical composition and also their motion, via the Doppler shift.

Astronomical Spectroscopy
Chemical elements can be detected in astronomical objects by their emission lines and absorption lines The shifting of spectral lines can be used to measure the redshift or blueshift of distant or fast-moving objects Helium was first discovered by spectral analysis of light from the Sun

Absorption Spectrum of the Sun
High resolution spectrum of the Sun showing thousands of elemental absorption lines (fraunhofer lines).

Hydrogen Absorption Spectrum

Origins of different Spectra

The Sun - A Nuclear Reactor
Energy is coming from the star’s core through fusion. Gravity is pulling the star towards its centre. When the star is born, the energy pressure outwards equals the gravity.

Formation of Deuterium as per The Big Bang

Hydrogen Fusion in Stars
Hydrogen fusion is the power source for the star. Hydrogen gets turned into Helium, and releases energy.

Nuclear Fusion to form Helium - another possible mechanism

Nucleosynthesis in a Massive Star

Spectral Classes of Stars
Astronomers want to classify the stars based on some property. Here the stars are classified by their mass. The lifetime of a star depends on the mass – higher mass stars live much less time. You can remember the class names – “Oh Be A Fine Girl, Kiss Me”

The Hertzsprung-Russell diagram.
Once a measurement of the star’s temperature and brightness have been made, they can be plotted on this diagram. Temperature correlates with colour. Red stars are cooler than blue stars. Our Sun is yellow. Most stars lie on the Main Sequence – a band on this graph where all the stars which are fusing hydrogen lie.

The classes also tell us what elements the star is made of.
These spectra show absorption lines which come from elements in the star’s atmosphere.

Illustrates cumulative absorption spectra observed by Hubble.
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