Wave-particle duality
- Light waves can behave like particles, i.e. photons, and waves
- This phenomenon is called the wave-particle nature of light or wave-particle duality
- Light interacts with matter, such as electrons, as a particle
- The evidence for this is provided by the photoelectric effect
- Light propagates through space as a wave
- The evidence for this comes from the diffraction and interference of light in Young’s Double Slit experiment
- Diffraction occurs when waves pass through a gap or around an obstacle and the waves spread out
Light as a particle
- Einstein proposed that light can be described as a quanta of energy that behave as particles, called photons
- The photon model of light explains that:
- Electromagnetic waves carry energy in discrete packets called photons
- The energy of the photons are quantised according to the equation E = hf
- In the photoelectric effect, each electron can absorb only a single photon - this means only the frequencies of light above the threshold frequency will emit a photoelectron
- The wave theory of light does not support a threshold frequency
- The wave theory suggests any frequency of light can give rise to photoelectric emission if the exposure time is long enough
- This is because the wave theory suggests the energy absorbed by each electron will increase gradually with each wave
- Furthermore, the kinetic energy of the emitted electrons should increase with radiation intensity
- However, in the photoelectric effect none of this is observed
- If the frequency is above the threshold and the intensity of the light is increased, more photoelectrons are emitted per second
- Although the wave theory provided good explanations for phenomena such as interference and diffraction, it failed to explain the photoelectric effect
Comparison of wave theory and particulate nature of light
The wave theory of light suggests... | This is wrong because... |
Any frequency of light can give rise to photoelectric emission if the exposure time is long enough | Photoelectrons will be released immediately if the frequency is above the threshold for that metal |
The energy absorbed by each electron will increase gradually with each wave | Energy is absorbed instantaneously - photoelectron are either emitted or not emitted after exposure to light |
The kinetic energy of the emitted electrons should increase with radiation intensity | If the intensity of the light is increased, more photoelectron are emitted per second |