Final answer:
Quantum explanation for the photoelectric effect is needed because classical physics couldn't explain the frequency-dependent energy of emitted electrons, the lack of time delay in emission, and low-intensity light's inability to eject electrons.
Step-by-step explanation:
The experimental observations that require a quantum explanation for the photoelectric effect are the following:
- The emission of electrons from a metal surface when exposed to light of a certain frequency.
- The dependence of the kinetic energy of emitted electrons on the frequency of the incident light.
- The absence of a time delay between the incidence of light and the emission of electrons.
- The inability of low-intensity light to cause the emission of electrons, regardless of how long the light is shone on the metal surface.
Each of these observations contradicts classical physics, which predicted that the number of electrons emitted and their kinetic energy would be associated solely with the intensity of the light, rather than its frequency. Einstein's explanation introduced the concept of light as being composed of photons, particles with quantized energy levels proportional to their frequency. This resolved the paradox by explaining that only photons with enough energy (above a certain threshold frequency) can eject an electron, with any excess energy contributing to the electron's kinetic energy.