Which observation provided albert einstein the clue that he needed to explain the photoelectric effect

The observation that provided the clue that Albert Einstein needed to explain the photoelectric effect was as below, " The energy of the photo-electrons does not depend on the intensity of light falling on the metal but on the frequency of the light." In other words, the energy of electrons being knocked out of metal is dependent on the frequency or wavelength of light and not on the intensity of light.

Answer) by studying proton collisions hope this : )

Idon’t know what the answer is i wish i could