As we know Maxwell explained to us the wave nature of electromagnetic radiation which is helpful for us to know about the interference, diffraction, refraction, deviations and etc of it. But that doesn't mean Maxwell succeeded to explain everything he wanted to there are concepts like natural emission of radiation from hot bodies, Photoelectric effect, electrons give out from the metal bodies when the electromagnetic wave hits it, and line spectra of atoms were left out of by the theory of Maxwell. The following all concepts were proved and fulfilled by Planck's Quantum theory.
The Quantum Planckstar -
As said above every object when struck by electromagnetic wave or heat it starts emitting a certain amount of radiation it is just like this, when a solid body is heated at a certain temperature starts changing color and emit certain radiation, and this changes every time as the temperature keeps on increasing. With that said according to the black body radiation principle by Planck, it is clear that the frequency of the radiation emitted by the black body totally depends on the intensity of the heat ( temperature ). The radiation is increased with the increase in wavelength and as time passes on there is a decrease in radiation with the increase in wavelength. These are some of the things that Maxwell failed to cover regarding radiations.
E = h v
E - Energy of the radiation
h - Planck constant ( 6.626 x 10^-34 Js)
v - Frequency of the radiation