Molecules at a double slit

The double slit experiment is one of the best known experiments in physics. It has been realized with photons, electrons, neutrons, atoms and large molecules.

A source emits objects that traverse two neighboring slits on the way towards the detector.

Try to predict what the distribution of particles on the detector could look like?

When an ensemble of particles of high mass or high velocity traverses the double slit, you’ll find a random pattern of single particles straight behind the two openings. This looks like…


For waves we would expect an interference pattern as a superposition of two diffracted partial waves behind the two slits.


A quantum interference pattern reveals both properties, of waves and particles in a single image. Like for classical particles we can identify individual objects, for instance using fluorescence or surface probe microscopy. But even when the individual particles travel independently from each other through the apparatus, the ensemble image is that of a wave.


Extra: What does the wave function signify?

The following applet shows the intensity distribution of a wave behind two neighboring slits in a screen. You can open or close either slit A or slit B or both and monitor the intensity distribution. Vary the wavelength to see its influence.

Wavelength \(\lambda\) - +
Distance A-B - +
Slit opening - +
Slit A
Slit B
Extra: Diffraction image behind the double slit.

Experimental challenge: Molecules at the Doubleslit

Go to the lab and follow the instructions. When you are done, return to this page and continue.