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Photons (light quantum)

Max Planck provided the foundation for the quantum theory with the law of rays (Strahlungsgesetz). According to classical physics radiation occurs in undefined small portions. According to quantum physics radiation occurs only as a multiple of the number h·n, with n being the frequency of the radiation and h being a natural constant, the so-called Planck‘s (quantum) constant: Its value is 6,63·10-34Js, its dimension (energy · time) is called effect.

In 1905, Albert Einstein put into words the light quantum hypothesis, which states that light of the frequency n consists of parts, called photons, with energy E =h·n .

The idea that light is made up of parts was in stark opposition to the theory of waves and raised the question if these two ideas could be somehow reconciled.