Much of Lord Rayleigh's research concerned the behavior of all sorts of waves. One of the waves produced by earthquakes is named for him. In 1871 he explained why the sky is blue, because of the preferential scattering of short wavelength blue sunlight in the atmosphere. The phenomenon is known as Rayleigh scattering today.
As previously noted, Rayleigh's Nobel Prize-winning work was in the discovery of the element argon. In his home laboratory, he discovered that nitrogen prepared from air was slightly heavier than nitrogen prepared from nitrogen-containing compounds. With the chemist William Ramsay, he found that the reason for this was an unknown gas that makes up approximately 1% of the atmosphere. They named the gas argon, from the Greek word for "inert", because they found that it would not react with anything. Ramsay then went on to find most of the noble gases, such as neon ("new"), krypton ("hidden"), and xenon ("stranger"), as well as isolating helium on earth, which previously had been known only from spectral lines on the sun.
What was possibly Rayleigh's greatest contribution to science came after the discovery of argon, and came about from a theory that failed to agree with experiment. Rayleigh attempted to find the spectrum of blackbody radiation, an equation that would describe the relative intensities of the various frequencies in the spectrum. Because the astronomer James Jeans made a few corrections to Rayleigh's derivations, the equation they found is known as the Rayleigh-Jeans formula, and it is hopelessly incorrect, as they were well aware, as for instance it predicts that the total energy density of a blackbody should be infinite no matter what its temperature is. This failure in their theory showed that something was fundamentally wrong with the whole structure of classical physics on which it was based. This failure of classical physics led Max Planck in 1900 to propose that light be emitted only in discrete packets which he called quanta, whose energy is proportional to the frequency of the emitted quantum. This produced a correct blackbody radiation formula, and formed the underpinnings of modern quantum mechanics.
However, Rayleigh, after a lifetime devoted to classical physics, never really accepted these new directions which became the underpinnings of modern physics.