How the "quantum of action" was forced on Physics 
''It was an act of desperation'' / Max Planck /
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In 1900, two formulas were known for thermal radiation ( Rayleigh–Jeansfor for long and Wien's for short wavelengths) and that is seemed strange to physicists. One of Planck's colleagues showed him these ''strange'' formulas . . . and Planck found a fairly simple solution. . . . Planck united together these two different formulas and then divided them. He was himself very surprised when the result was found correct. . . . The formula worked perfectly and gave correct radiation intensity at every frequency. Maybe some thousands of physicists were satisfied with this result as the end of searching. But the great Max Planck asked himself: ''Why is the formula correct? '', ''What does the result mean?''. And in his Nobel Lecture given on 2 June 1920, Planck described how he made his discoveries: '' . . . eventually after some weeks of the hardest work of my life, light entered the darkness, and a new inconceivable perspective opened up before me. ... '' The result was – quantum of action (as energy multiplied time: h=Et). The coefficient (h) was neither in the Rayleigh–Jeansfor nor in the Wien's formulas. Planck took unit (h) as in some books are written: "intuitively, instinctively, phenomenological" . . . . Planck didn't explain ''where did the (h) come from?'' . . . He took (h) '' from heaven '' . . . as later he said it was ''an act of desperation'' . . . Many - many years (more than 20 years) Planck tried to explain (h) from the classical point of view . . . but without success . . . At first the physics community received the (h) skeptically ( ''h'' contradicted all fundamental classical laws) . . . but later the existence of (h) was proved by other physicists (it fit the experimental data well) . . . . but even today we cannot say that we know its (h) essence.  From 1900 to 2020 the quantum (h) enigma is still with us.
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Yes i enjoy 'Quantam of Solace' too, great film

Yep. Quantum theory works perfectly and gives correct radiation intensity at every frequency. Who would have thought that new theories become accepted merely because they are congruent with a greater range of observations than previous ones?