Longer waves should not fit into the blackbox.

This means that there should be FEWER waves of long wavelengths (low frequencies).

Hence, most of the energy (as you heat the blackbody) SHOULD go into the shorter wavelengths.

This, in effect, was not observed.

Lower frequencies were found in a HIGHER proportion. And after a frequency cutoff, no higher frequencies were even found!

Planck to the rescue

Planck prosposed that instead of thinking of ‘equipartition of energies’, what if one thought of a special distribution of frequencies. Only frequencies that absorbed a constant ‘quanta’ of energy were allowed.

E  = hf –> Where f is the frequency.

Lower frequencies should actually participate more in the energy absorption process, as only a slight amount of energy is required to be completely absorbed. The higher the frequency, the more energy would be required, making higher frequencies rarer. This is what was observed experimentally as well.