How do physicists define light?
Some of the Greek philosophers, but also Galileo, speculated about the nature of light. But it was only in the last few centuries that this became apparent to physicists, so that today we can define light.
Light - a historical foray
- Greek philosophers were already concerned about light. For example, the Pythagoreans assumed that every visible object was constantly releasing a stream of light particles. But also Galileo tried to fathom the nature of light in the last years of his life - unfortunately without success.
- At the end of the 17th In the early 20th century, the physicist Newton assumed that light consists either of particles, so-called corpuscles, or of waves. However, he initially ruled out the wave theory, as the particle hypothesis could be better reconciled with his mechanistic worldview at the time. This is how corpuscular theory could explain the fact that light propagates in a straight line, is reflected and refracted.
- Around the same time as Newton, the Dutch naturalist Huygens was also concerned with the nature of light. According to his ideas, light should propagate in the form of waves. These waves should be transmitted through an invisible, weightless substance, the ether, which exists everywhere in space.
- However, both models made different statements about the speed of light in substances, for example in water. However, this speed could not be measured so precisely with the devices available at the time to differentiate between the two models "particle" or "wave".
Light is a wave - Young proves it
- With his double slit experiment, the physicist Young was able to prove in 1802 that light spreads like a wave.
- In this experiment, light hits a (small) double slit. Behind it, overlays (and not shadows) are formed, which can only be explained if one understands light as a wave process.
- With this, the bitter controversy about particle or wave propagation finally seemed to be over.
- However, both the wave appearance and the wavelengths are so small that they cannot be observed with the naked eye, unlike water waves. This tiny size was ultimately the reason why the wave nature was recognized very late.
Photons and waves - simple explanation of light
In physics there is hardly anything that is more interesting and less for the layman ...
Light also has to be a particle
- However, Newton was probably not wrong with his idea of particles.
- At the beginning of the last century, physicists had to give light a particle character in order to explain the photo effect, for example.
- Already Max Planck was able to find many of the contradicting results, for example from the Atomic physics, only explain by postulating (i.e. assuming!) that light consists of tiny packages of energy. He called these quanta. They were absorbed or given off when they interacted with atoms.
- And in 1905 Albert Einstein declared the photo effect with the same assumptions. Only with quantum theory could he show how high-energy radiation releases electrons from a metal (Nobel Prize 1921).
How do physicists "define" light today?
- Of course you can't "define" a phenomenon like light, but you can develop models that show how light behaves in certain situations and experiments.
- Even if this sounds contradictory, one has to assume today that light is not only a wave phenomenon, but also has the character of a particle, i.e. occurs in individual portions. These are called light quanta or photons.
- An experiment shows that this contrary view, also known as wave-particle dualism, applies single light quanta can be generated, which are sent one after the other through a double-slit apparatus. The wave phenomena then occur.
- Between the two physically opposing phenomena particle and wave there is a formal one "Bridge", namely Planck's quantum of action h, which is between the frequency (or wavelength) of light and the energy which mediates photons. The following applies: E = h x f.
- Even more: Even for typical particles such as electrons or protons, modern physics has been able to show that they also have the character of waves when they propagate. This is how matter waves are created when particles of atomic size are sent through a double slit.
So how is light to be "defined"? Wave or particle, neither or both together? Even if the theoretical physics can mathematically describe the behavior of light and its phenomena, light is still a mystery. Both models are justified. The physicist Richard Feynman once answered this question: "Maybe a third one!". Light obeys the laws of quantum mechanics, which allow a duality of particles and waves.
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