Max Planck (Max Karl Ernst Ludwig Planck) Describing Colour Changes by Coupling the Physics of Heat and Light. Planck’s revolutionary idea of describing light statistically rather than as a continuous wave seeded the birth of quantum physics. In physics, Planck’s law describes the spectral radiance of electromagnetic radiation at all wavelengths from a black-body at temperature. Max Planck originally proposed his law in 1900 in an attempt to improve upon an expression proposed by Wilhelm Wien which fit the experimental data at low wavelengths (high frequencies) but deviated from it at high wavelengths (low frequencies).
The Colour of Heat
Many things glow when they are heated up. For example, the rings on a stove turn red, reaching temperatures of hundreds of degrees Celsius, while volcanic lava glows more fiercely – perhaps, orange, yellow or even white hot – as it reaches temperatures approaching a thousand degrees Celsius. As the temperature of a substance increases, it glows first red, then yellow and eventually white. The light looks white because more blue light has been added to the existing red and yellow. This spread of colours is described as a black-body curve.
Scientists in the 19th century were surprised to find that the light emitted when objects were heated followed the same pattern, irrespective of what substance they were testing. Most of the light was given off at one particular frequency. When the temperature was raised, the peak frequency shifted to bluer (shorter) wavelengths, moving from red through yellow to blue-white.
Frequency Peaks at a Single Colour
Even though scientists had measured black-body graphs, they could not explain why the frequency peaked at a single colour. Wilhelm Wien, Lord Rayleigh and James Jeans had all worked out partial solutions. Wien had described the dimming at bluer frequencies mathematically while Rayleigh and Jeans explained the red spectrum. However, both of these formulae failed to explain the full picture.
In trying to understand black-body radiation, Max Planck combined the physics of heat and light together. Planck’s insight was to treat electromagnetic radiation in the same way as thermodynamics experts treated heat. Just as temperature is the sharing of heat energy among particles, Planck described light by allocating electromagnetic energy among a set of electromagnetic oscillators.
Planck scaled the energy of each electromagnetic unit with frequency, such that E=hv, where E is energy, v is light frequency and h is a constant scaling factor now known as Planck’s constant.
Although Planck’s oscillators were originally just a construction for working out the mathematics of his law, his formulation effectively planted the seed that would grow to become quantum theory.