The photoelectric effect provided experimental evidence that contradicted the wave theory of light. Three primary observations are as follows:
Electron emission from a material surface in the photoelectric effect only occurs when the incident light's frequency surpasses a specific threshold value, irrespective of its intensity. Wave theory posits that light's energy disperses over time, implying that heightened intensity (energy delivered per unit time) should eventually suffice to eject electrons, even at lower frequencies. The wave model fails to account for the necessity of a minimum frequency (threshold frequency) for electron release. This phenomenon is explicable by quantum theory, which posits that light comprises discrete energy packets (photons) with energy directly proportional to frequency, expressed as \( E = hu \).
In the photoelectric effect, electrons are ejected instantaneously upon illumination with light of sufficient frequency, exhibiting no discernible delay. Wave theory suggests that light, as a wave, would require energy accumulation over time to dislodge an electron, thus predicting a delay in emission. However, experimental findings demonstrate immediate electron emission when light of the appropriate frequency is incident, supporting the interaction of light as discrete particles (photons) rather than a continuous wave.
Wave theory predicts that light intensity, related to wave amplitude, should influence the kinetic energy of emitted electrons, with higher intensity imparting more energy. In contrast, the photoelectric effect shows that increasing light intensity (while maintaining frequency above the threshold) boosts the number of emitted electrons but does not alter their kinetic energy. This observation aligns with the particle theory of light, where each photon's energy is constant and frequency-dependent, not intensity-dependent. Intensity affects the quantity of photons striking the surface, not the energy carried by individual photons.