Imagine two walls :

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Now how many

vibrational modes can you fit between the two walls ? This is the question that was asked when people tried to calculate the energy spectrum that a heated object
should emit. Back in the day before quantum mechanics
the world was considered

continuous. This means that
you can divide the seperation between the walls into
an infinite set of modes. Each mode contributes equally
to the

energy and as your number of modes diverges so does the energy that you expect to see coming out of the heated object. This is

the ultraviolet catastrophe. Planck
set a limit to the smallest size of the modes. He

quantized
the system. This was the first introduciton of

quantization into physics. He did it in

1905