bit about hydrogen and helium but my excuse is that they make up the bulk of the visible matter in the Universe. Similarly wavelengths, which, like frequency, can be used to describe the behaviour of different kinds of solar energy from X-rays to radio waves. You do not have to be a geek to appreciate such matters, witness a useful mnemonic for the relationship between wavelength and frequency to be found in one of the tales of diplomatic life by Lawrence Durrell: If there is anything worse than a soprano, said Antrobus judicially, as we walked down the Mall towards his club, it is a...