His theory is fascinating. When you heat up a black metal poker, why does the colour change? It goes through a series of changes, from black, to red, to yellow, white, and blue as the temperature (thermal radiation) increases. Planck constructed a mathematical model in which the thermal radiation was connected to a particular vibrational frequency (harmonic oscillator).
He proposed that each oscillator produced a specific number of units of energy – i.e. that each temperature related to an exact (or quantised) amount of energy that gave off a specific light. This is opposed to a random, arbitrary amount of energy emitted as the object warms up.
So to summarise, the increase in temperature led to increased emittance of energy and a larger proportion of the energy is focused on the violet end of the light spectrum. The most important part of this is the fact that the energy was quantised – i.e. it increased in specific, set, tiny, incremental amounts. This was groundbreaking.
Planck did not realise the full implications of his theory, but his paper sparked many further hypotheses. At around the same time Albert Einstein had developed his Theory of Relativity (which described the behaviour of objects in motion at high speed). Planck’s work inspired Einstein to look in closer depth at the behaviour of light. He discovered that light could also be quantised and he described the specific-particles-of-light as individual photons. But why did this matter? Einstein’s theory that light could behave as a particle was controversial.
It was in opposition to the current idea that light behaved as a wave, which had been established thanks to decades of experiments in refraction, diffraction and interference, which showed that light tended to move in waves and crests like ripples on a lake, it could be bent, could bounce around corners and more. You may remember some of these experiments with light from school.
After much scepticism, Einstein’s theory was eventually accepted as true, along with the other evidence that it behaved as a wave. This is the second most important thing to take on board. Light can behave as both a particle and a wave.
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