This part of the story begins with the simple fact that hot objects glow. The hotter an object gets, the brighter it glows. Also, its color changes from red to orange, to yellow, and so on, eventually becoming blue (or beyond). The problem starts when we try to understand and describe how this glow of light occurs.

An ideal object of this type is called a black body. The classical theory of the light produced by a hot black body was developed by Lord Rayleigh and Sir James Jeans, using the type of thermodynamic theory that Maxwell used to describe gases like air. The Rayleigh-Jeans theory gave a great description of the light observed at the infrared and red end of the light spectrum, but rapidly disagreed with experimental results for the blue - ultraviolet end of the light spectrum. Rayleigh-Jeans' result predicted a rapidly increasing intensity of light from a black body as the light wavelength grew shorter ( red - yellow - green - blue - ultraviolet). This is what became known as the ultraviolet catastrophe. Luckily for us, the amount of light actually emitted by a black body drops rapidly as the wavelength gets shorter, so we are not severely burned by all of that UV predicted by Rayleigh and Jeans!

Now, we need to understand why the theory of Rayleigh and Jeans failed for short wavelengths. The solution to this puzzle will yield an independent corroboration of Einstein's idea about light packets, and actually came first. The first, correct theory of blackbody radiation was devised by Max Planck, who did not believe his result to be correct for many years.