Double Slit Interference and Wave-Particle Duality

Double Slit Interference for Waves

• When waves pass through a barrier with two open slits, a screen behind this barrier will show the separation between minima and maxima of this wave’s interference pattern (caused by the wave passing through the barrier slits)

Double Slit Interference for Particles

• If we imagine this same setup, but now instead of a wave passing through the slits, we imagine particles, we can imagine that there should be no interference – the particles should appear on the screen behind the barrier in a pattern identical to their incident direction going through the slit.

The Experiment

• In Young’s Double Slit experiment, Thomas Young created a double slit interferometer setup using light as the source

• At the time, it was accepted that light behaved as a particle due to Newton’s experiments

• Young hypothesized that light acted like a wave, and as such by his experiment he should see an interference pattern caused by the double-slit barrier

The Experiment cont’d

• Young was right: he observed an interference pattern caused by the light source.

• Confirming his hypothesis, this showed that light did act as a wave – a radical idea that contradicted Newton

• Over time, the idea that light acted as a wave became accepted and the idea of light as a particle was discarded

Light as both a particle and a wave

• Albert Einstein reintroduced the idea of light as a particle – the photon

• Einstein believed that light behaved both like a particle and as a wave

• Einstein used the photoelectric effect to demonstrate this: when incident light is shone on a metal, the metal emits electrons

• If light only acted as a wave, then its amplitude or wavelength should affect the number of emitted electrons (by conservation of energy)

Light as both a particle and a wave

• However, only the frequency of the incident light causes electrons to be emitted

• Einstein suggested that light behaved as a particle, by the photon, and also as a wave when there was a large collection of photons

• This hypothesis paved the way for the development of quantum mechanics in physics