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

Energy of electrons depends on light's frequency, not intensity.

Explanation:

The photoelectric effect was discovered by physicist Henrich Hertz. Albert Einstein, in 1905, modernized the concepts of this effect when he realized that the energy of electrons depends on the frequency of light, not on intensity.

The discovery of this effect occurred between 1886 and 1887 by Henrich Hertz, who used classical physics to explain it. The classical concepts made the conception of this phenomenon insufficient, giving way to the modern concepts proposed by Albert Einstein in 1905.  Among Einstein's proposals are the quantization of energy, that is, for the occurrence of immediate electron ejection from the surface, the radiation energy (electromagnetic waves) would be concentrated in packages (photons) and not distributed over the wave (classical prediction). It also demonstrated that the speed at which electrons are ejected does not depend on the amount of photons emitted, but on the frequency of these photons.