In this mini-tutorial we consider the famous photon double-slit experiment that Feynman discusses in his lecture series. An applet is set up to calculate interference patterns as a function of the slit width. The sum over paths method is used to demonstrate at what slit widths (expressed as a multiple of the wavelength) the transition occurs in the photon behaviour from naively expected ray tracing to true wavelike behaviour.
The diagram illustrates the basic set-up. We keep the source-aperture- detector distances relatively large in order to explore the regime where interference becomes unimportant due to a large slit width. Also the separation between the slits is chosen to be large compared to the slit width. An intriguing question which we would like to answer is what it is that makes small slit widths so special, and what we mean by small.
The applet allows one to step through the detector height Y.
Observe the contributions to the total photon arrival amplitude from both slits. When the detector is in the line of sight from the source the photon intensity does not fall to zero. How does this come about? Why is the intensity pattern asymmetrical with respect to the middle position between the two slits? For fixed slit separation ask yourself how the interference pattern should depend on the slit width (keeping in mind the Fraunhofer single-slit diffraction results). Then run the simulation to test your prediction.