Double Slit Experiment


The double-slit experiment is a seminal demonstration in quantum physics that highlights the wave-particle duality of matter and energy. In its basic form, a beam of particles (such as photons, electrons, or even atoms) is directed toward two parallel slits cut into an opaque barrier. A detector screen is placed on the other side of the barrier to record the arrival of the particles.

Classical Expectation

If the particles were simply classical objects (like tiny marbles), we would expect to see two distinct bands of accumulation on the detector screen, corresponding to the locations of the two slits.

Quantum Reality: The Interference Pattern

However, when the experiment is performed, an interference pattern is observed on the detector screen. This pattern consists of alternating bright and dark bands, indicating that the particles are behaving like waves. The waves passing through each slit interfere with each other, creating areas of constructive interference (bright bands) and destructive interference (dark bands).

Key Interpretations

The double-slit experiment has far-reaching implications for our understanding of the nature of reality:

  • Wave-particle duality: The experiment demonstrates that light and matter can exhibit properties of both waves and particles, depending on how they are observed.
  • The role of observation: The very act of attempting to observe which slit a particle passes through changes the outcome of the experiment. When an observer is present to determine the particle's path, the interference pattern disappears, and the particles behave like classical objects. This raises profound questions about consciousness and its role in influencing physical reality.
  • Superposition: Before being measured, the particles are said to exist in a state of superposition, meaning they exist in all possible paths and states simultaneously.

Historical Development

  • Thomas Young (1801): Young first demonstrated the wave nature of light using a similar double-slit setup.
  • Later Experiments: The experiment has been repeated with electrons, atoms, and even molecules, consistently demonstrating the wave-like nature of matter.


  • Delayed-choice experiment: This version of the experiment suggests that a particle can "decide" whether to behave as a wave or a particle even after it has passed through the slits.
  • Quantum eraser experiment: This variation demonstrates that information about a particle's path can be erased after it has passed through the slits, restoring the interference pattern.


The double-slit experiment is a cornerstone of quantum mechanics. It challenges our classical understanding of the world and has inspired numerous philosophical debates and interpretations about the nature of reality and the role of the observer.