Wave particle duality is the idea that all particles can be described as waves as well as particles. Experiments with light showing it acting as a wave and particle have helped us to understand and develop this theory.
In 1672, Newton suggested that light was a stream of particles. His laws of motion as well as observations of refraction and reflection helped to back up this idea. However, 6 years later, Huygens opposed this idea and predicted light actually was a wave. From this, he predicted that light would travel slower in a denser medium, proved correct in later experiments.
Over 100 years later, in the early 1800s, Thomas Young conducted his famous double-slit experiment to show light acting as a wave. For a time, it looked as if Newton had been wrong but then in the early 20th century, Planck used the photoelectric effect to show light as a particle. Einstein later wrote further equations to improve this theory.
Young’s Double Slit Experiment
Thomas Young conducted the experiment by shining a monochromatic light source through two small slits, the light diffracted after passing through the slits then interference occurred. This interference led to a series of bright and dark sections being made on the screen.
Interference is when two or more waves collide and a new wave pattern is then created. The waves can interfere constructively, greater amplitude, or destructively, which reduced the amplitude. The bright fringes observed are caused by constructive interference and the dark fringes are from destructive interference.
The Photoelectric Effect
The photoelectric effect is the emission of electrons from a surface of a material when electromagnetic radiation is incident on the surface.
In order for these electrons to be released, the frequency of the electromagnetic wave must be above what is known as the threshold frequency. This will be different for each material, the energy required is called the work function.
Intensity of a wave will not increase the energy of released electrons, only the number of electrons released. This suggests that light acts as particles (photons) because the energy of the wave is dependent on photon energy (calculated from frequency). Also, intensity will increase amount of photons incident on surface, therefore increasing the amount of electrons released.
The energy of a photon is defined as planck’s constant multiplied by the frequency of the wave. The photon energy is equal to the work function of the metal plus the max kinetic energy of released electrons:
hf = Φ + KEmax
This can be used to find the maximum kinetic energy of the released electrons from a material if frequency and work function are known.
Electrons as Waves
De Broglie suggested that all particles had a wavelength associated with it. The wavelength could be found by the formula:
λ = h/mv
Where h=Planck’s constant, m=mass and v=velocity
This suggests all particles have wavelengths but because most have such high mass, these wavelengths are negligible. Waves can be though of as probability functions, showing the probability of finding a particle in a certain place.
OCR AS/A Level Physics by Mike O’Neill