The Two Slit Experiment

The two slit experiment dates from the 19^th century when it was used to demonstrates the wave property of light. In recent years it has been used to illustrate a very weird aspect in quantum mechanics.

PART 1 – THE BASIC TWO SLIT EXPERIMENT

Step	Description	Notes
1	Imagine a source of light shining against a screen with two slits in it.
2	As the light hits the screen, it passes through the slits. These act as two sources of light which then shine onto a second, solid screen.
3	When the light hits the second screen, we observe an interference pattern, made up of light and dark fringes.
4	The light spreads out from the slits in the first screen (diffraction) and the two sets of waves overlap. As they overlap, they interfere with other, creating the fringe pattern that we observe on the second screen.
5	This demonstrates that photons behaves as waves.	Not just photons, but any quantum-sized particle, such as atoms or electrons.
6	The same effect is observed with water waves, using ripples in a water tank instead of light.

PART 2 – A TWO SLIT EXPERIMENT WITH PARTICLES

Step	Description	Notes
7	Imagine the screen with two slits is turned so that it is horizontal	Particles of sand will always behave as particles, and never as waves, because they are not quanta, like atoms, electrons or photons.
8	This time we use grains of sand to represent particles. Each grain will run through one slit or the other.
9	Two piles of sand are observed to form, one pile beneath each slit.
10	Now let’s repeat the experiment but using atoms instead of sand.  The atoms are streamed through the two slits, just like the light source in step 1. We instinctively think of atoms as being particles, so we would expect to observe two bands on the back screen (like the two piles of sand)
11	What we actually observe is an interference pattern, identical to step 3. This demonstrates that the atoms have gone through the slits, behaving as waves, not as particles.

PART 3 – A TWO SLIT EXPERIMENT WITH INDIVIDUAL PARTICLES

Step	Description	Notes
12	This time, instead of sending a stream of atoms, we send the atoms one at a time.  We wait until we observe a spot appears on the back screen where the atom has landed, then fire the next atom and so on. In this illustration, six atoms have been fired. No obvious pattern is observed yet.
13	When we have eventually fired millions of atoms, one at a time, over many hours, we observe the same interference pattern that we saw in step 11, gradually appear.  The atoms arrive on the back screen as localised points, but each atom behaves as if it is part of a wave.

PART 4 – A TWO SLIT EXPERIMENT WITH INDIVIDUAL PARTICLES AND A DETECTOR

Step	Description	Illustration
14	If we repeat the above experiment with an atom detector next to the top slit, we observe a different pattern gradually appear. We observe two lines rather than the wave pattern (fringes). This is what we would expect to see if the atoms behaved as particles (like the grains of sand in step 9).
15	This demonstrates that the atoms are behaving as if they are particles, just because they are being detected and measured.
16	The conclusion is that if we don’t detect the atoms, we observe the atoms behaving as waves. But if we do detect and measure individual atoms, they behave as particles.	There is currently no agreed explanation for this!
17	If we leave the detector in place but switch it off, we observe the wave pattern as in step 11, because the atoms are no longer being detected.
18	This demonstrates that we observe the wave behaviour of the atoms (see step 11) as long as we don’t detect individual atoms. The detection device causes them to behave as particles rather than waves.	Two important properties of the quantum world are revealed in these experiments: Wave-Particle Duality and Wave Function Collapse

 The lecture from which the above is taken can be found here.