Question

Mention any three features of results of experiment on photoelectric effect which cannot be explained using the wave theory of light.

Hint:

Einstein's explanation of the photoelectric effect relies on the particle theory of light, where light is treated as quanta (photons).

Answer 1

The photoelectric effect experiment demonstrated several features that cannot be explained using the wave theory of light. Here are three key features: 

1. Discrete Threshold Frequency:

In the photoelectric effect, it was observed that no electrons are emitted from the surface of a material unless the frequency of the incident light exceeds a certain threshold value, regardless of the light's intensity. According to wave theory, the energy of light is spread out over time, so increasing the intensity (i.e., the energy delivered over time) should eventually provide enough energy to release electrons, even at lower frequencies. However, the wave theory fails to explain why a minimum frequency (threshold frequency) is required to release electrons, and this observation can only be explained by the quantum theory, which proposes that light has discrete packets of energy (photons) with energy proportional to frequency \( E = h\nu \).

2. Instantaneous Emission of Electrons:

In the photoelectric effect, electrons are emitted instantly when light of sufficient frequency strikes the surface, with no noticeable delay. According to the wave theory, if light were purely a wave, the energy would accumulate over time to free an electron. Therefore, there should be a delay before electron emission begins. However, experimental results show that electron emission occurs immediately when light of the correct frequency is incident, indicating that light interacts as discrete particles (photons), not as a continuous wave.

3. Effect of Intensity on Electron Kinetic Energy:

Wave theory suggests that the intensity of light should affect the energy of the emitted electrons, as intensity is related to the amplitude of the wave, and higher intensity should impart more energy to the electrons. However, in the photoelectric effect, increasing the intensity of the light (while keeping the frequency constant above the threshold) increases the number of emitted electrons, but does not affect their kinetic energy. This behavior is explained by the particle theory of light, where the energy of each photon is fixed and proportional to the frequency, not the intensity. Intensity simply affects the number of photons striking the surface, not the energy of each photon.