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+<sect1 id="ai-blackbody">
+
+<sect1info>
+
+<author>
+<firstname>Jasem</firstname>
+<surname>Mutlaq</surname>
+<affiliation><address>
+</address></affiliation>
+</author>
+</sect1info>
+
+<title>Blackbody Radiation</title>
+<indexterm><primary>Blackbody Radiation</primary>
+<seealso>Star Colors and Temperatures</seealso>
+</indexterm>
+
+<para>
+A <firstterm>blackbody</firstterm> refers to an opaque object that
+emits <firstterm>thermal radiation</firstterm>.  A perfect
+blackbody is one that absorbs all incoming light and does not
+reflect any.  At room temperature, such an object would
+appear to be perfectly black (hence the term
+<emphasis>blackbody</emphasis>).  However, if heated to a high
+temperature, a blackbody will begin to glow with
+<firstterm>thermal radiation</firstterm>.
+</para>
+
+<para>
+In fact, all objects emit thermal radiation (as long as their
+temperature is above Absolute Zero, or -273.15 degrees Celsius),
+but no object emits thermal radiation perfectly; rather, they are
+better at emitting/absorbing some wavelengths of light than others.
+These uneven efficiencies make it difficult to study the interaction
+of light, heat and matter using normal objects.
+</para>
+
+<para>
+Fortunately, it is possible to construct a nearly-perfect blackbody.
+Construct a box made of a thermally conductive material, such as
+metal.  The box should be completely closed on all sides, so that the
+inside forms a cavity that does not receive light from the
+surroundings.  Then, make a small hole somewhere on the box.
+The light coming out of this hole will almost perfectly resemble the
+light from an ideal blackbody, for the temperature of the air inside
+the box.
+</para>
+
+<para>
+At the beginning of the 20th century, scientists Lord Rayleigh,
+and Max Planck (among others) studied the blackbody
+radiation using such a device.  After much work, Planck was able to
+empirically describe the intensity of light emitted by a blackbody as a
+function of wavelength.  Furthermore, he was able to describe how this
+spectrum would change as the temperature changed.  Planck's work on
+blackbody radiation is one of the areas of physics that led to the
+foundation of the wonderful science of Quantum Mechanics, but that is
+unfortunately beyond the scope of this article.
+</para>
+
+<para>
+What Planck and the others found was that as the temperature of a
+blackbody increases, the total amount of light emitted per
+second increases, and the wavelength of the spectrum's peak shifts to
+bluer colors (see Figure 1).
+</para>
+
+<para>
+<mediaobject>
+<imageobject>
+<imagedata fileref="blackbody.png" format="PNG"/>
+</imageobject>
+<caption><para><phrase>Figure 1</phrase></para></caption>
+</mediaobject>
+</para>
+
+<para>
+For example, an iron bar becomes orange-red when heated to high temperatures and its color
+progressively shifts toward blue and white as it is heated further.
+</para>
+
+<para>
+In 1893, German physicist Wilhelm Wien quantified the relationship between blackbody
+temperature and the wavelength of the spectral peak with the
+following equation:
+</para>
+
+<para>
+<mediaobject>
+<imageobject>
+<imagedata fileref="lambda_max.png" format="PNG"/>
+</imageobject>
+</mediaobject>
+</para>
+
+<para>
+where T is the temperature in Kelvin.  Wien's law (also known as
+Wien's displacement law) states that the
+wavelength of maximum emission from a blackbody is inversely
+proportional to its temperature.  This makes sense;
+shorter-wavelength (higher-frequency) light corresponds to
+higher-energy photons, which you would expect from a
+higher-temperature object.
+</para>
+
+<para>
+For example, the sun has an average temperature of 5800 K, so 
+its wavelength of maximum emission is given by:
+
+<mediaobject>
+<imageobject>
+<imagedata fileref="lambda_ex.png" format="PNG"/>
+</imageobject>
+</mediaobject>
+</para>
+
+<para>
+This wavelengths falls in the
+green region of the visible light spectrum, but the sun's continuum
+radiates photons both longer and shorter than lambda(max) and the
+human eyes perceives the sun's color as yellow/white.
+</para>
+
+<para>
+In 1879, Austrian physicist Stephan Josef Stefan showed that
+the luminosity, L, of a black body is proportional to the 4th power of its temperature T.
+</para>
+
+<para>
+<mediaobject>
+<imageobject>
+<imagedata fileref="luminosity.png" format="PNG"/>
+</imageobject>
+</mediaobject>
+</para>
+
+<para>
+where A is the surface area, alpha is a constant of proportionality,
+and T is the temperature in Kelvin. That is, if we double the
+temperature (e.g. 1000 K to 2000 K) then the total energy radiated
+from a blackbody increase by a factor of 2^4 or 16.
+</para>
+
+<para>
+Five years later, Austrian physicist Ludwig Boltzman derived the same
+equation and is now known as the Stefan-Boltzman law. If we assume a
+spherical star with radius R, then the luminosity of such a star is
+</para>
+
+<para>
+<mediaobject>
+<imageobject>
+<imagedata fileref="luminosity_ex.png" format="PNG"/>
+</imageobject>
+</mediaobject>
+</para>
+
+<para>
+where R is the star radius in cm, and the alpha is the
+Stefan-Boltzman constant, which has the value:
+
+<mediaobject>
+<imageobject>
+<imagedata fileref="alpha.png" format="PNG"/>
+</imageobject>
+</mediaobject>
+</para>
+
+</sect1>