Double slit interference experiments can show wave-like behavior for both waves and particles. Young's double slit experiment with light showed an interference pattern, demonstrating that light acts as a wave even though it was thought to be a particle based on earlier experiments. Later, Einstein introduced the idea that light behaves as both a particle (photon) and wave, based on observations from the photoelectric effect. This helped establish the principle of wave-particle duality in quantum mechanics.

1. Double Slit Interference and
Wave-Particle Duality

2. 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)

3. 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.

4. 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

5. 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

6. 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)

7. 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