On This Day: 30th June

It would be fantastic to commemorate some of the greatest scientists and/or their findings on each day in history that I blog, hence here is a new section of my website called “On This Day…”.


For the first of these, I would like to commemorate the death of John William Strutt, aka Lord Rayleigh (died aged 76 on 30th June 1919, Essex). A number of significant breakthroughs are owing to Rayleigh’s studies, such as the aptly-named Rayleigh scattering (which, if your child ever asks about the blueness of the sky, you can simply say “Rayleigh scattering” and hope to get away with it with no explanation…) as well as winning the 1904 Physics Nobel Prize for discovering argon. But the primary reason for my choice of this gentleman to mark today’s figure of fame is because the Rayleigh-Jeans spectral scattering law is still a fresh wound in my memory, due to having recently finished a module in my degree course called ‘Quantum Phenomena’… (Oh I’m only allowing for some light-heartedness in an otherwise bleak post; I did genuinely enjoy that module – in fact I’d go as far as saying it was my favourite in year 1!). But I digress. The said law, B_\lambda(T) = \frac{2 c k T}{\lambda^4}, is an empirical description of the temperature-specific spectral radiance of electromagnetic radiation from a blackbody (otherwise known as a surface which absorbs/emits perfectly at all wavelengths of light). Derived from classical experiments, it is slightly wrong as it incorrectly predicts infinite levels of radiance at low frequencies (ever heard of the Rayleigh Catastrophe?) and this error led to one of the most important discoveries in 20th century physics. Simultaneously, Max Planck was working on blackbody radiation, leading him to his own equivalent derivation, using the constant ‘h’ and also the Boltzmann constant ‘k’: B_\lambda(T) = \frac{2 h c^2}{\lambda^5}~\frac{1}{e^\frac{hc}{\lambda kT}-1},. Being the despised half-blood “math-physicist” * that I am, I won’t go into the details of the derivation of how Planck’s law magically (i.e. binomially) becomes the Rayleigh-Jeans law within classical limits (mainly due to my formatting incapabilities… I make it my mission to learn LaTeX soon enough! Although here you can download a pretty darn good explanation) – but Planck’s law eventually made the quantum theory possible, with the realisation of light’s particle-wave duality and the very slim probability that a blackbody would radiate at the smallest wavelengths (corresponding to the greatest energies), thus making the overall spectral emittance at those low wavelengths also quite small – and hey ho, the law agrees with experiment!

The maze of science quite fascinating. Here, Rayleigh was on the right path. But it took a collaboration with Mr. Jeans to turn the  λ−4 dependence into a fully derived law with constants. Yet they still hit a closed door and only Planck held the key to open it – and many other quantum doors to follow.


* “Math-physicists” tend to be disowned by either department here at Warwick. The mathematicians hate me. The physicists hate me. But me, I’m not “mudblood”…. I’m half-blood and the best of both worlds 😀

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