Blackbody+Radiation


 * Blackbody Radiation **

It is not just the Sun that emits electromagnetic radiation. All bodies o, and over the whole frequency range of the spectrum. The distribution over frequency depends on the body’s temperature. If a solid is hot enough, it will glow visibly, but as it cools its glow will diminish as the longer wavelength radiation beyond the visible dominates. This does not mean that it ceases to emit visible light but simply that the intensity of the light will be too weak for us to see. Of course, all matter also absorbs and reflects radiation falling on it. Which wavelengths are absorbed and reflected defines the colour of everything we see.

Physicists in the second half of the 19th century were very interested in the way a particular type of warm object, known as a black body, emits radiation. Black bodies are so called because they are perfect absorbers of radiation and do not reflect light or heat. Of course, a blackbody must somehow dispose of all the energy it absorbs; otherwise its temperature would be infinite. Therefore, it radiates off its heat over all possible wavelengths. The wavelength of the most intense radiation depends of course on the temperature of the blackbody.

The radiation emitted matched that of the predictions of earlier theories at wavelengths longer that visible light. However, the failure of the theory (Rayleigh’s theory) manifested itself in a curve which shot up to infinity in the ultraviolet region of the spectrum.

Max Planck’s suggestion was this: if a black body is ultimately composed of vibrating atoms, then the energy they give off depends on their frequency of vibration. This would mean that the higher their frequency, the more energy they would emit. The key point was that such oscillators would only have certain modes of vibration and their frequencies would have to ramp up in definite steps rather than smoothly. Therefore the energy given off would only have certain values since not all possible energies would be allowed. That is, the energy would come in quanta. This was a radical departure from Maxwell’s EM theory in which energy is regarded as continuous.

(Planck did not regard all energy as ultimately composed of irreducible tiny lumps. That had to wait five more years for Einstein’s photoelectric effect paper to be published in 1905)

Think of a ball rolling down a flight of stairs and giving up its potential energy in jumps instead of continuously, as when it rolls down a smooth slope. This is contrary to our everyday experiences in the aspect that energy is not a continuous concept, but can only be quantified as simple multiples of the Planck constant.