Spectra of Light
Spectrum, in optics, the arrangement according to wavelength of visible, ultraviolet, and
infrared light. An instrument designed for visual observation of spectra is called a
spectroscope; an instrument that a photographs or maps spectrum is a spectrograph. The
typical spectroscope is a combination of a microscope and a prism. The prism breaks the light
into its spectra components (by differential refraction) which is then magnified with a
microscope.
Spectra may be classified according to the nature of their origin, i.e., emission or absorption.
An emission spectrum consists of all the radiations emitted by atoms or molecules, whereas in
an absorption spectrum, portions of a continuous spectrum (light containing all wavelengths)
are missing because they have been absorbed by the medium through which the light has
passed; the missing wavelengths appear as dark lines or gaps.
The spectrum of incandescent solids is said to be continuous because all wavelengths are
present. The spectrum of incandescent gases, on the other hand, is called a line or emission
spectrum because only a few wavelengths are emitted. These wavelengths appear to be a
series of parallel lines because a slit is used as the light-imaging device. Line spectra are
characteristic of the elements that emit the radiation. Line spectra are also called atomic
spectra because the lines represent wavelengths radiated from atoms when electrons change
from one energy level to another. Band spectra is the name given to groups of lines so closely
spaced that each group appears to be a band, e.g., nitrogen spectrum. Band spectra, or
molecular spectra, are produced by molecules radiating their rotational or vibrational energies,
or both simultaneously.
What is Spectroscopy?
Spectroscopy is a very important tool in astronomy. It is detailed study of the light from an
object. Light is energy that moves through space and can be thought of as either waves or
particles. The distances between the peaks of the waves of light are called the light's
wavelength. Light is made up of many different wavelengths. For example, visible light has
wavelengths of about 1/10th of a micrometre - ten thousand wavelengths would be the width
of a penny.
Spectrometers are instruments that spread light out into its wavelengths creating a spectrum.
Within these spectra, astronomers can study emission and absorption lines that are the
fingerprints of atoms and molecules. An emission line occurs when an electron drops down to
a lower orbit around the nucleus of an atom and looses energy. An absorption line occurs
when electrons move to a higher orbit by absorbing energy. Each atom has a unique spacing
of orbits and can emit or absorb only certain energies or wavelengths. This is why the location
and spacing of spectral lines is unique for each atom.
Astronomers can learn a great deal about an object in space by studying it’s spectrum, such as
its composition (what its made of), temperature, density, and it's motion (both it's rotation as
well as how fast it is moving towards or away from us).
There are three types of spectra that an object can emit: continuous, emission and absorption
spectra. The examples of these types of spectra shown below are for visible light as it is
spread out from purple to red, but the concept is the same for any region of the
electromagnetic spectrum.
Continuous spectra
Continuous spectra (also called a thermal or blackbody spectra) are emitted by any object that
radiates heat (has a temperature). The light is spread out into a continuous band with every
wavelength having some amount of radiation. For example, when sunlight is passed through a
prism, it's light is spread out into it's colours.
A continuous visible light spectrum
Absorption spectra
If you look more closely at the Sun's spectrum, you will notice the presence of dark lines.
These lines are caused by the Sun's atmosphere absorbing light at certain wavelengths,
causing the intensity of the light at this wavelength to drop and appear dark. The atoms and
molecules in a gas will absorb only certain wavelengths of light. The pattern of these lines is
unique to each element and tells us what elements make up the atmosphere of the Sun. We
usually see absorption spectra from regions in space where a cooler gas lies between us and a
hotter source. We usually see absorption spectra from stars, planets with atmospheres, and
galaxies.
Detailed image of our Sun's visible light spectra
The absorption spectra of hydrogen - can you see this pattern in the solar spectrum above
this image?
(Hint: hydrogen is the most abundant element in the sun - look at the darkest lines).
Emission spectra
An emission spectra occurs when the atoms and molecules in a hot gas emit extra light at
certain wavelengths, causing bright lines to appear in a spectra. As with absorption spectra,
the pattern of these lines are unique for each element. We can see emission spectra from
comets, nebula and certain types of stars.
The emission spectra of hydrogen
In practice, astronomers rarely look at spectra the way they are displayed in the above images.
Instead they study plots of intensity or signal versus wavelength. These plots show how much
light is present or absent at each wavelength. A peak in the plot shows the position of an
emission line and dip shows where an absorption line is. The spacing and location of these
lines are unique to each atom and molecule.
The shape of the continuous spectra (often referred to as the continuum) on a plot is
dependent on temperature and motion of the emitting gas. In this simple plot it is shown as a
flat line - in reality it is usually a curved line. Also, many of the real data plots you will see
have the wavelength or frequency on a logarithmic scale.
Examples of spectra
Hydrogen:
Helium:
Carbon:
[ Loads more on:
http://www.northallertoncoll.org.uk/Physics/Learning%20Resources/Module%201/spectra%2
0web%20links.htm ]
Thus, emission spectra are produced by thin gases in which the atoms do not experience
many collisions (because of the low density). The emission lines correspond to photons of
discrete energies that are emitted when excited atomic states in the gas make transitions back
to lower-lying levels.
A continuous spectrum results when the gas pressures are higher. Generally, solids, liquids,
or dense gases emit light at all wavelengths when heated.
An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the
gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in
the same direction as the absorbed photon, this gives rise to dark lines (absence of light) in the
spectrum.
Hydrogen Emission and Absorption Series
The spectrum of hydrogen is
particularly important in
astronomy because most of the
Universe is made of hydrogen.
Emission or absorption processes
in hydrogen give rise to series,
which are sequences of lines
corresponding to atomic
transitions, each ending or
beginning with the same atomic
state in hydrogen. Thus, for
example, the Balmer Series
involves transitions starting (for
absorption) or ending (for
emission) with the first excited
state of hydrogen, while the
Lyman Series involves
transitions that start or end with the ground state of hydrogen; the adjacent image illustrates
the atomic transitions that produce these two series in emission.
Because of the details of hydrogen's atomic structure, the Balmer Series is in the visible
spectrum and the Lyman Series is in the UV.
Web sites:
Spectra
http://www.colorado.edu/physics/2000/quantumzone/
http://www.ipac.caltech.edu/Outreach/Edu/Spectra/spec.html
http://home.achilles.net/~jtalbot/data/nebula/NGC2440.html
Bohr Model of the atom
http://csep10.phys.utk.edu/astr162/lect/light/bohr.html
http://zebu.uoregon.edu/~js/glossary/bohr_atom.html
http://www.physicslessons.com/exp28b.htm
Quantum theory notes:
http://science.howstuffworks.com/atom8.htm
Problems of Rutherford theory:
http://www.symonds.net/~deep/stuff/qp/chap04.php
Bohr applet & notes:
http://www.mhhe.com/physsci/astronomy/applets/Bohr/frame.html
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/BohrModel/BohrModel.html
Loads of revision notes:
http://www.jcphysics.com/toolbox_indiv.php?sub_id=23