If light consists only of photons

Light from photons

Often photons are thought of as the components that make up light. In fact, it is true that the quanta that are absorbed by atoms from light are identical to the elementary particles called photons. However, one should not think of it as if light were a stream of small particles or wave packets. A photon is neither a classical particle nor a wave packet. In this section I would like to briefly show how the photon image can be correctly understood physically.

Elementary waves

The first step in describing real light mathematically is to break it down into elementary waves. Real light always consists of different wavelengths that can be broken down into the colors of the rainbow in a glass prism. A similar decomposition of a wave is also mathematically possible. Any wave can be broken down into an infinite number of elementary waves, the superposition of which results in the original wave. An elementary wave is just a model of thought that can be described mathematically. It is infinitely long and has an exactly defined wavelength. Although elementary waves are actually infinitely long and therefore cannot describe real waves, one can mathematically define superpositions that describe finitely long wave trains.

Quantization to Photons

In the next step, the elementary waves are quantized into photons. Quantizing means here that the elementary waves cannot have any total energy. Your energies can only increase and decrease in steps (quanta). Max Planck first came to this realization when he wanted to mathematically describe black body radiation. Later it turned out that quantization is a common phenomenon not only in Planck's special calculation, but in the entire quantum world (i.e. in microscopic dimensions).

The photons of light "inherit" the properties of the elementary waves: They are infinitely long and have a fixed frequency and wavelength. But they also have a fixed energy that only depends on their frequency, and they also carry momentum and angular momentum.

That the infinitely long photons have a limited energy (thus their energy density is zero) seems a bit surprising at first. Mathematically this is calculated by adding the so-called limes (Limit value) forms. So one calculates photons that are "locked up" in a certain area of ​​space and thinks this area is getting bigger and bigger. It shows that the photon energy does not grow if you let the room become infinitely large.

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Last change: 05.09.2005