Failure of Classical Wave Theory

According to classical wave theory,

• Intensity of a wave is the energy incident per unit area per unit time.
• Energy carried by an electromagnetic wave is proportional to the square of the amplitude of the wave.

Classical wave theory cannot explain the first 3 observations of photoelectric effect.

1. Existence of the threshold frequency

• Since energy of the wave is dependent on the square of its amplitude, the classical wave theory predicts that if sufficiently intense light is used, the electrons would absorb enough energy to escape. There should not be any threshold frequency.

2. Almost immediate emission of photoelectrons

• Based on classical wave theory, electrons require a period of time before sufficient energy is absorbed for it to escape from the metal. Accordingly, a dim light after some delay would transfer sufficient energy to the electrons for ejection, whereas a very bright light would eject electrons after a short while. However, this did not happen in photoelectric effect.

 3. The independence of kinetic energy of photoelectron on intensity and the dependence on frequency

• According to classical wave theory, if light of higher intensity is used, the kinetic energy of an ejected electron can be increased. This is because the greater the intensity, the larger the energy of the light wave striking the metal surface, so electrons are ejected with greater kinetic energy. However, it cannot explain why maximum kinetic energy is dependent on the frequency and independent of intensity.