Rayleigh
(1842-1919)
John William Strutt was knighted the 3rd Baron Rayleigh in 1873 on the death of his father and was elected to the Royal Society the same year.
In 1879 he succeeded
James Clerk Maxwell as the second Cavendish professor at the University of Cambridge.
Rayleigh's contributions to science included two that were particularly important for
quantum mechanics. The first was his 1900 distribution law for radiation, developed further with Sir James Jeans in 1905, which describes the distribution of energy (or intensity) as a function of wavelength (or frequency), especially for long wavelengths.
Rayleigh showed that the distribution of energy is inversely proportional to the fourth power of the wavelength λ. This disagreed with the 1896 derivation of a distribution law by
Willy Wien, as a function of radiation frequency ν. Frequency ν and wavelength λ are related by the equation
λν = c.
Wien's distribution law fit the experimental data very well for high frequencies, but the Rayleigh-Jeans law diverges strongly at those short wavelengths. When
Max Planck was shown the long-wavelength data in 1900, within a few days he found a new mathematical formula that interpolated between the Wien values at high frequencies and the Rayleigh values at long wavelengths.
The new Planck radiation distribution law, and especially Planck's controversial derivation of the law, by assuming that the energy of radiating oscillators should be limited to discrete amounts equal to
hν, where
h is Planck's new constant he called the "quantum of action."
Albert Einstein in 1905 showed that Planck's assumption of discrete oscillator energies must mean that the radiation emitted (and absorbed) by oscillators must not be
continuous radiation but
discrete particles of energy that Einstein called "light quanta" (today's "photons"). Planck opposed this idea for many years, though he is often given credit for it in modern texts.
Diffraction of Light and Resolving Power
Another of Rayleigh's great contributions to physics was his criterion for the minimum angular resolution of optical devices like microscopes and telescopes. The limit is imposed by the diffraction of light at the sharp edge of an object or by the size
d of a hole or slit through which light of wavelength
λ must pass.
Diffraction of light causes the
interference fringes that are the essential phenomenon in the
two-slit experiment.
Rayleigh found the minimum angular resolution
θ is proportional to the ratio of the wavelength of light
λ to the size
d of the slit.
θ = 1.22λ/d
We can see the reason for this as the
destructive interference of waves coming from the two edges of the slit at the angle
θ, such that the path of one wave is out of phase, with its crest exactly cancelling the other's trough.
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