Light and the Quantum World


Photon Idea

On the previous pages, I was describing light from the nonquantum point of view. But quantum is modern, quantum is fashionable, so I think it is time to say a few words about the quantum description of light.

On the last couple of pages I think I convinced you that light is an electromagnetic wave described by Maxwell's equations. Until the end of 19th century, this was the universally accepted description of light.

At the beginning of the 20th century, the situation changed. Physicists started to make new discoveries related to light. In most of these cases, the new phenomena could not be explained on the basis of the wave theory. For example, experimentalists collected lots of data on how the amount of energy radiated from a heated cavity (called a black box) depends on the wavelength of the emitted radiation. Maxwell's equations could not explain the observations.

As we saw, everything about electromagnetic waves is hidden in Maxwell's equations. Therefore it must be possible to explain the experimental results of the black-body radiation from Maxwell's equations. Everybody tried very hard.

They failed.

Then, in 1900, Max Planck, a German physics student, was taking an exam on electricity and magnetism taught by Maxwell. One of Maxwell's questions was to explain the black-body radiation. Did Planck pass?

Please find out on the next page.

Photon Idea

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Last updated: June 18, 1999 AP