The Double Slit ExperimentInterference Wave Patterns and Why they Appear

Particle Patterns vs. Wave Patterns in Single Slit Experiments

When a particle of some kind is emitted from a source behind a barrier with a single slit, a pattern appears that directly relates to the size of the slit.

When a wave is emitted from a source behind a barrier with a single slit, the wave passes through the slit and radiates outwards to form a similar pattern, with the highest intensity being in the center,

Particle Patterns vs. Wave Patterns in Double Slit Experiments

When a particle is emitted from a source behind a barrier with a double slit, a pattern appears that directly relates to the size and position of the two slits.

However, when a wave is emitted from a source behind a barrier with a double slit, the wave passes through the slits and radiates outwards but forms a pattern of many lines as opposed to just the two from the slits.

Why is this?When radiating waves from two sources interfere with each other, they create regions of maximum and minimum pressure where they collide. These points are illustrated in red and blue below:

Point of highest intensity is still in the center:

Multi-Bar Light Intensity Pattern from Wave Interference

Because of these previously mentioned points of maximum and minimum pressure, when the pattern is projected onto a 2-dimensional surface such as in the diagram below, a pattern appears of alternating lighter and darker regions representing these regions of higher and lower pressure.

Where n=0, the center, on the diagram, this is the highest intensity as there have been more interference points before reaching the surface.

For increasing n moving away from the center of the source, the intensity decreases as there are less interference points between before the waves reach the surface.

The Double Slit Experiment in the Quantum World

Armed with the knowledge of the difference between particle and wave patterns in double slit experiments, you are now ready to appreciate the inconclusive results of an amazing physics experiment that baffles scientists to this day:

https://www.youtube.com/watch?v=DfPeprQ7oGc

Constructive Thinking A scientist sets up a barrier experiment to test quantum

particles for wave interference patters. Below are the results at increasing times:

1. Describe what is happening on the black recording surface as time passes. Why is the pattern becoming stronger later

on?

2. Is this a double slit or a single slit experiment? How do you know?

Constructive Thinking: Answers

As time passes, waves are interfering with one another and creating points of high pressure which we see as the white points on the graph. It is difficult to tell earlier on because there aren’t enough points to make out a pattern yet, but as they accumulate, we begin to see regions of higher pressure forming in bands, with the lower pressure dark areas in between.

This is a double-slit experiment. We know this because if there were only one slit there would only be one wave and therefore it would not have anything to interfere with. It would instead leave a pattern similar to that or a beam of particles (eg. a single band in the center)

References Hawkes, (2014). Physics for Scientists and Engineers: An

Interactive Approach. Custom ed. Toronto: Nelson Education Ltd.

(2006). Dr Quantum - Double Slit Experiment. Available at: https://www.youtube.com/watch?v=DfPeprQ7oGc.

Blacklight Power (2015). Double Slit Explanation of Classical Electron Diffraction. Available at: http://www.blacklightpower.com/theory-2/theory/double-slit/.

Young's Double Slit Experiment. Available at: http://ipodphysics.com/prop-of-light-youngs-double-slit.php.