Why did the classical phisics fail to explain the black body radiation.However,The modern phisics did ?

Question

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Answer 1

Ah, the "ultraviolet catastrophe"

It was the first substantial indication of the existence of the quantum world.

In the 1890's black body radiation was found not to match the predictions of Newton and Maxwell, disastrously so at the ultraviolet end of the spectrum.
Max Planck, in addressing this, found the first opening into incredible new phenomena. A whole new view of the world.

See below

Answer 2

Quantum theory was needed to explain it correctly.