We saw in the last post that energy that can be transferred by conduction and convection, two very different molecular processes. But it is the third mechanism of heat transfer that is the most surprising.
In radiation, the transfer of energy is not a molecular process at all. Instead, the energy is carried as an electromagnetic wave, that is a wave consisting of oscillating electric and magnetic fields. The fields are self-perpetuating (and mutually perpendicular) because the changing electric field induces a magnetic field and the changing magnetic field induces an electric field.
The discovery of electromagnetic radiation emerged late in the 19th century. From Maxwell’s theory of electromagnetism, it was realised that light itself consists of an electromagnetic wave: however, it took Einstein to realise in 1905 that electromagnetic waves travel from the sun to earth through a vacuum i.e. do not need a medium in which to travel (unlike conduction or convection).
The rate of radiation from the sun (or any body hotter than its surroundings) is proportional to the fourth power of its temperature i.e. is extremely sensitive to temperature. Radiation also depends on a property of the body known as emissivity. Emissivity is a measure of how well a material emits radiation and is determined by atomic processes within the body. For this reason, a good emitter is also a good absorber, if it is placed in an environment where it is cooler than its surroundings (a perfect absorber is called a blackbody, because it will absorb all light incident on it). The opposite of a good absorber or emitter is a reflector, an object which can neither absorb nor emit heat. Polished metals and bright materials tend to be goodish reflectors: for this reason white clothes are worn when playing cricket and tennis in hot countries (they reflect both heat and light, keeping the player cool and easy to see).
A hot body does not radiate energy at a particular frequency, but at all frequencies – from waves of high energy and frequency (gamma rays) to low-energy ones (radiowaves). The low energy waves have low frequencies but long wavelengths since the wavelength of a wave is inversely proportional to its frequency. The full range of frequency (or wavelength) of radiation is called the electromagnetic spectrum. One of the great unifying moments in physics occured when it was realised that radiowaves, microwaves, infra-red heat, visible light, ultra-violet light, X-rays and gamma rays are all versions of the same thing – they are simply electromagnetic waves of different frequencies (and wavelengths).
Even a blackbody body dies not radiate equally at all frequencies. The distribution of radiation vs frequency (i.e. the spectrum of radiation) depends on the temperature. A body at extremely high temperatures will radiate predominantly at high frequencies, while a body at very low temperatures will radiate predominantly at much lower frequencies. Below is a picture of the emisson spectrum of a blackbody, measured at several different temperatures.
This spectrum is of great interest in fundamental physics, because it turns out that it cannot be predicted using the laws of classical physics. In the early years of the 20th century, Planck and Einstein showed that the blackbody spectrum could only be explained if it was assumed that light behaves as a stream of discrete particles in some circumstances. This duality i.e. light behaving as wave in some circumstances and as a stream of particles in others forms the basis of the famous quantum theory (and was later found to be true of matter as well as of radiation i.e. the tiniest ‘particles’ of matter such as electrons can exhibit wave behaviour!)
In cosmology, the cosmic background radiation is a faint background radiation that permeates the entire universe. It is radiation that is almost as old as the universe itself, dating back to the time after the Big Bang when the universe had expanded and cooled just enough for the first atoms to form, allowing radiation to travel freely (up to this point in time radiation was scattered by the different particles) . Do you think the cosmic background radiation will be hot or cold? At what frequency do you think it is observed? What kind of spectrum might be expected?
3 responses to “Introductory physics: radiation”
Nice. One of the most surprising things about radiation, classically speaking, is that there is a pressure associated with it, and beyond that, actual energy and momentum. This is one of the strong arguments that electric and magnetic fields are real, and are not just a useful mathematical tool.
indeed. You could argue that a good deal of modern physics has the em field as its cornerstone – from the experiments of Hertz to blackbody radiation, from uantum electrodymanics to the electroweak inteaction
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