Quantum Mechanics
Interference: Double-slit

Schematic sett-up of two-slit experimentThe numerical simulation of the two-slit experiment is shown in the next animation. Initially the quantum particle is represented by a Gaussian wave packet of width sigma. The dimensions of the two slits are given in the schematic diagram.

Simulation result for two-slit experiment

Intensity recorded by a detector placed far away from a sourceThe maxima and minima in the probability distribution are a direct proof of the presence of interference. The intensity I (theta) recorded by a detector placed far away from a source characterizes the interference pattern. The intensity I (theta) is called the angular distribution. The simulation yields an angular distribution that is in excellent agreement with Angular distribution the Fraunhofer diffraction pattern of a double slit. If you were to use a pencil to draw this pattern you would barely see the difference with the simulation result.

The following animations illustrate the effect of changing the dimensions of the double slit. As in the case of the single slit the qualitative features of the diffraction pattern Schematic set-up two-slit experiments do not depend on the length w of the double slit. Remember that an experiment with a single particle does not produce an interference pattern. It is only after repeating the experiment with the single particle many times that an interference pattern emerges.

s = lambda, d = 2*lambda Simulation results for s = lambda, d = 2*lambda
s = lambda, d = 4*lambda Simulation results for s = lambda, d = 4*lambda
s = lambda, d = 6*lambda Simulation results for s = lambda, d = 6*lambda
s = 2*lambda, d = 5*lambda Simulation results for s = 2*lambda, d = 5*lambda

Schematic set-up for asymmetric double-slit experimentImagine that you were given the double slit shown on the right. If you would use this device in your experiment would you expect that the interference pattern

  1. will be asymmetric?
  2. is wiped out completely?
  3. does not change at all?

The answer to this question can be found by carefully looking at the following animation.

Simulation result for asymmetric two-slit experiment