Friday, December 7, 2012

Lord Rayleigh, Biological Physicist

I am a big fan of Victorian physicists. Among my heroes are Faraday, Maxwell, and Kelvin. Another leading Victorian was John William Strutt, also known as Lord Rayleigh (1842-1919). Russ Hobbie and I mention Rayleigh in the 4th edition of Intermediate Physics for Medicine and Biology, in the context of Rayleigh Scattering. In Chapter 15 on the interaction of x-rays with matter, we write
“A photon can also scatter elastically from an atom, with none of the electrons leaving their energy levels. This (γ, γ) process is called coherent scattering (sometimes called Rayleigh scattering), and its cross section is σcoh. The entire atom recoils; if one substitutes the atomic mass in Eqs. 15.15 and 15.16, one finds that the atomic recoil kinetic energy is negligible.
In Rayleigh scattering, the oscillating electric field in an electromagnetic wave exerts a force on electrons. These electrons are displaced by this force, and therefore oscillate at the same frequency as the wave. An oscillating charge emits electromagnetic radiation. The net result is scattering of the incident wave. If the electrons are free, this is known as Thomson scattering. If the electrons are bound to an atom, and the frequency of the light is less than the natural frequency of oscillation of the bound electrons, then it is known as Rayleigh scattering. Light scattering is complicated when the wavelength is similar to or smaller than the size of the scatterer, because light scattered from different regions within the particle interfere. However, Rayleigh scattering assumes that the wavelength is large compared to the size of the scatterer, so interference is not important.

Rayleigh scattering not only plays a role in the scattering of x-rays, but also is responsible for the scattering of visible light. The Rayleigh scattering cross section varies as the 4th power of the frequency, or inversely with the 4th power of the wavelength. When we look at the sky, we see the scattered light. Since the short wavelength blue light is scattered much more than the long wavelength red light, the sky appears blue.

Lord Rayleigh made other important contributions to physics. For example, he wrote an influential book on the Theory of Sound, and he won the Nobel Prize in 1904 for his discovery of the element argon. He succeeded Maxwell as the Cavendish Professor of Physics (see this video: to learn more).

Was Rayleigh a biological physicist? Yes! Rayleigh was one of the first to explain how we localize sound. His Duplex Theory suggests that we can determine the direction a sound came by sensing the arrival time difference at each of our two ears for low frequencies, and sensing the intensity difference between the ears for high frequencies.

Lord Rayleigh was born 170 years ago this fall (November 12, 1842). J. J. Thomson studied under Rayleigh, and Ernest Rutherford studied under Thomson. Previously in this blog, I described how I am descended, academically speaking, from Rutherford. This means Lord Rayleigh is, again academically speaking, my great-great-great-great-great-great grandfather.

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