A box that absorbs all light that comes into it and generates endless energy? How Max Planck changed our understanding of physics.
Imagine a black box with a hollow cavity and with a very tiny hole in it. Into that hole, we shoot light. What will happen? This box is a technical realization of a black body, meaning the light we shoot into the hole will reflect as long as almost all the light gets absorbed. Only a small amount of thermal energy leaves the box again.
Before I will explain the black body problem, I need to give you a bit of background. When you heat an object, it emits light because it thermally agitated the electrons on the surface, “and electrons being accelerated and decelerated radiate light”. So what is a black body? That is the next question we have to clarify. And also why is this hollow cavity so important for Planck’s thesis on quantum mechanics? A black body is something “ that reflected nothing and absorbed all the light falling on it (hence the term black body, because the object would appear perfectly black as it absorbed all light falling on it). When you heat a black body, it would radiate, emitting light”. We can make experiments on this matter by using the hollow cavity as an approximation of the black body. The hole in the hollow cavity needs to be small because we want to keep a thermal balance in the box. The last thing you need to know before we go on is that before Planck, physicists believed that light would radiate continuously. We will see, however, that this is not the case.
The thing with the hollow cavity was that physicists could measure the spectrum of black body radiation after that black body was heated. No one could theoretically explain this spectrum. Two physicists Rayleigh and Wien tried to find a formula for the black body radiation spectrum, but what they postulated was fatal. According to the Rayleigh formula, the energy of the radiated light would expand with smaller wavelengths to an extent where more energy comes out of the black body than what was imported. That, however, is not possible.
Just as a side note, wavelength, and frequency have the following relation: Frequency= speed of light/ wavelength.
The Wien formula, however, worked fine for high frequencies (small wavelengths), but it failed for low frequencies. I sketched the black body radiation spectrum for you here:
On the y-axis, we have the Energy density and on the x-axis the Frequency. You can see the spectrum of a black body for two different temperatures: T1 and T2. Again, “nobody was able to come up with a theoretical explanation for the spectrum of light generated by the black body”.
And then Max Planck came along in 1900. “The black-body problem was a tough one to solve, and with it came the beginnings of quantum physics. Max Planck came up with a radical suggestion — what if the amount of energy that a light wave can exchange with the matter wasn’t continuous, as postulated by classical physics, but discrete?”. Planck introduced his famous formula E= Nhf, with N being a natural number, h a constant, and f the frequency. This formula says that Energy releases in small units, not in a continuous spectrum.
The constant h is Planck’s quantum of action and describes the smallest unit of discrete energy we have in the universe. All radiated energy is a multiple of this constant, with the value of h= 6,626 070 15 · 10−34 Js. Planck could also find the right formula for the black body radiation spectrum by combining the Wien and Rayleigh-Jean Law. With Planck’s thesis, physicists had a new perspective of light as a duality of wave and particle. It was the beginning of the new big thing in physics: quantum theory.
If you are interested in quantum mechanics, read my next stories and be good!
References:
https://en.wikipedia.org/wiki/Black-body_radiation
