The Ultraviolet Catastrophe and Blackbody Radiation

Article Summary

In this article we develop the theoretical background of electromagnetic radiation and blackbody radiation together with a rigorous logical argument to explain the phenomenon (and consequences), of ‘‘The Ultraviolet Catastrophe’.

Electromagnetic waves are waves that are made up of oscillating electric and magnetic fields. The electric field and the magnetic field oscillate at right angles to each other, and at right angles to the direction of wave propagation and energy transmission. There are different kinds of electromagnetic waves: visible light, ultraviolet waves,and x-rays are just a few examples. The different types of electromagnetic waves differ from each other in wavelength and in frequency. The wavelength is the spatial distance between the maximum points of the oscillating fields.The frequency is the amount of time it takes for one oscillation to occur – that is, for a single point on the wave to traverse one wavelength,which involves descending from a maximum point to a minimum point, and then rising to the maximum height again. Together, wavelength and frequency tell us the speed the wave is travelling at: the greater the distance between maxima, the faster the wave has to move through space if all its points are going to undergo one oscillation per second.

All electromagnetic waves move at the same constant speed, which means that waves with shorter wavelengths have higher frequencies.We therefore can line up the different kinds of electromagnetic waves on a spectrum,according to their wavelengths and frequencies. Electromagnetic waves carry energy, which is transmitted in the direction of wave propagation. We call the transmission of energy via electromagnetic waves electromagnetic radiation. The greater the frequency of a wave, the more energy it carries and transmit.

A blackbody is a physical object that perfectly absorbs and re-emits all kinds of electromagnetic radiation. A blackbody is to be contrasted with a white body, an object that perfectly reflects all electromagnetic radiation. Say, for instance, we have a black rubber ball.If we leave this black rubber ball out in the sunlight, which is comprised of a wide range of electromagnetic waves, our black rubber ball would absorb the radiation, ‘digest’ the radiation, and then re-emit a new assortment of electromagnetic waves types. We call the variety of electromagnetic waves absorbed the absorption spectrum of the blackbody, and the variety of electromagnetic waves emitted the emission spectrum of the blackbody. Like many concepts in physics, concept of a blackbody is idealized. There are no physical objects that perfectly absorb and emit electromagnetic radiation. Nevertheless, modelling objects as blackbodiesis extremely helpful when approximating the absorption and emission spectra of physical objects. To account for the fact that most physical objects are not true blackbodies, different materials are assigned numbers that express the extent to which the material behaves like a blackbody. This number is called the material’s emissivity coefficient.

You can access the next articles in this suite, by clicking the appropriate link below:

Stern-Gerlach Experiment 

Wave-Particle Duality

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Article written by:
Anisa Kureishi
Tutor