Evans SPH 4U1

Physics Grade 12

Unit 9: Quantum Theory

Note 6: Wave-Particle Duality


Reference: All light/Wave Chapters

Here is a compilation of your summaries..........................


Now let's summarize the all the particle and wave theories and experiments of light to show how light exhibits both particle and wave properties. Perhaps light is more complex than this, but for now we have to accept this wave-particle duality.


Wave Model:

Reflection: When waves strike a barrier, the angle of incidence is equal to the angle of reflection. Also the speed of the wave is unchanged.

Refraction and Partial Reflection: When waves pass from a deep region into a shallow region they are bent towards the normal and slow down. Some reflection also takes place.

Diffraction: Waves tend to bend when they pass through an opening or around the edges of an obstacle. Waves with longer wavelengths will diffract more.

Interference: Waves from 2 point sources produce nodal lines in the same way bright and dark bands are produces when light passes through a double slit.

Polarization: Light can be polarized which proved that light is a transverse wave.


Particle Model:

The original Newton's Particle model was effective in explaining reflection and dispersion but was weak in explaining.................

Partial-Reflection-Partial Refraction: Newton proposed that some particles hit the surface and get reflected and some hit the surface and get     refracted.

Diffraction: Newton thought light didn't travel around corners and he thought the effect around a barrier was just particles randomly colliding at the edges rather than outward spreading of waves.

Refraction: Newton believed that as light travels from one medium to another it encounters a net force at the boundary - however, this would have predicted that light would speed up in glass and water which is incorrect.


The Particle Model became effective in explaining...................

Blackbody Radiation: Scientists couldn't explain the ultraviolet catastrophe with the wave model and Planck proposed that atoms in a heated material vibrate with specific quanta of energy. Electromagnetic waves do not emit energy continuously.

Photoelectric Effect: Electrons are emitted from photoelectric materials when exposed to light of certain frequencies. Here light can be visualized as "particle-like" photons with energy. These photons give energy to the electrons of a photoelectric material causing them to be ejected.

Compton Effect: An elastic collision takes place when X-rays are scattered from free electrons. Here light (X-rays) can be visualized as "particle-like" photons possessing both energy and momentum.



Taylor's Experiment:  Demonstrated that the wave model determines the probability that a photon will be at a certain position in space at a given time.

deBroglie: He stated that any particle with momentum would have an associated wavelength (demonstrating more wave-particle duality).


Antoher Note: Bohr's Principle of Complementarity stated that although light has wave particle duality, in order to understand specific problems/experiments we must use either the wave theory OR the photon theory but not both. (Usually we use the wave theory when light is acting through a medium or space and we use the particle theory when light is interacting with matter).