Two slits

Imagine shining a beam of light through a tall narrow slit onto a screen. What you see on the screen is a tall narrow bar of light on the screen opposite the slit. It is a pretty easy thing to imagine- the light passes through the slit in a straight line and shines on the screen making a bright vertical bar.

Now imagine closing that slit and opening a second one just shifted a bit to the side of the first. Now you will see another vertical bar of light on the screen opposite the new slit. The vertical bar of light you see now is just like the first, but it is shifted to the side a bit because the slit has moved to one side.

Now, let's open both slits. You might now expect to see two vertical bars of light, one opposite each slit, but you don't. Instead what you find is a pattern of light and dark vertical stripes!

The stripes- which are called interference fringes- are easily explained by assuming that the light coming through the slits is a wave.

You remember that when two waves overlap they add together. Where a peak or trough meets another peak or trough you get a doubly tall peak or doubly deep trough, but when a peak meets a trough they cancel each other out and you get nothing.

When the light comes through two slits it is two separate waves, one coming from each slit. At certain points on the screen the two waves arrive with their peaks and troughs aligned, and you get brightness on the screen. At other places the waves arrive with the troughs of one lined up with the peaks of the other, and they cancel each other out, so you get a dark spot. There are hundreds of visual explanations of this on the Internet, so check out any of them if you can't imagine my written description.

The key thing to remember, however, is that when waves come through two slits they interfere with each other, reinforcing themselves in some places and cancelling each other out in others so you get alternating bars of light and dark where the waves meet.

Famously, the two slits experiment has been performed with fundamental particles, including electrons, protons, atoms, etc. And what is found is that the distribution of particles hitting the screen beyond the slits forms interference fringes. There are vertical areas where lots of particles land alternating with vertical areas where no particles land. The interference pattern happens even if the particles are fired through the slits one at a time. The only theory (currently) that accounts for this is that the particle has  wave-like property, and the particle's wave goes through both slits, and the two component of the particle's wave interfere with each other when they recombine on the other side of the slits.

This experiment works even when the distance between the slits is much greater than the size the particle appears to have when it manifests itself as a particle. This seems to suggest to some physicists that the particle is in its wavelike form when it is passing through the slits, and manifests in its much smaller point-like form when it hits the screen the other side of the slits. What 'really' happens is still anyone's guess.