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Black Radiation. .

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Black body radiation estimates of the Age of the universe gave a value of only 2 billion years, which was much less than the 5 billion year age of the Earth estimated from such measures as the abundance of radioactive minerals and their decay products in rocks.

As the temperature of a body is raised it radiates energy as shorter and shorter wavelengths. When it reaches about 600oC a dull red glow is emitted, at 2000oC, the temprature of an electric filament, a yellowish light is given out. Hotter still and the light appears bluer. All the colours comprise a mixture of wavelengths spreading about a maximum intensity. If this maximum depend only on the temprature and not on any property of the body we call the Black Body, because it has no colour of its own.

If the object were coloured, red say, when heated it would appear cyan. The reason for this is that a red body absorbs the all cours other than red when cool. All bodies come to equilibrium with their surroundings. When placed in a hot furnace the 'red' body will be absorbing blue and green so it will emit them, but because it does not absorb red it will not emit red. A black body on the other hand will absorb all wavelengths and so will emit them all as well. For practical work a hollow ceramic with a small hole in it acs as a black object when cold and can be heated without damage.

When such bodies are heated it is found that the maximum wavelength is determined by Wein's Law, Lambda maxT = a constant, 2.93 x 10 m K, and the total energy radiated per second is given by Stefan's Law, P = Theta AT4, where A is the surface area of the emitter and Theta is Stefans constant, 5.7 x 10-8 W m-2 K-4.

Black body radiation could not be accounted for by the theories of physics at the time. The statistics of the equilibrium of gas molecules in a container had been worked out by James Clerk Maxwell and is called Maxwellian distribution. One of the principles underlying this was the equipartion of energy, the energy of one molecule is the energy of the total divided by the number of molecules present, subject to fluctuation caused by mutual collisions. When the temperature was changed the peak of the distribution shifted. The same idea was tried out with black body radiation. The peak of the wavelength distribution moved when the temperature changed, and the shape of the distribution was similar to the Maxwellian one.

However there is one important difference; the number of gas molecules is finite, whereas the number of oscillators inside the black body is infinite. Equipartion of energy would predict that the radiation emitted would rise to higher and higher frequencies but with lower and lower intensities.

theory would predict that a black body should emit ultraviolet which it did not do. This became known as the ultraviolet catastrophe, (and would have been an X-ray and Gamma Gamma(y)-ray catastrophe had they been discovered at the time). It was Max Planck who solved the problem and introduced a new style of physics, quantum theory.

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