The two slit experiment, as emphasized by Feynman, exhibits the central mystery of quantum mechanics. While it's not surprising that waves diffracted through slits can create a static interference pattern on a distant screen, the fact that individual particles like photons or electrons can be made to interfere with themselves, is indeed shocking and mysterious.
From a colleague's web page:
The dark and light regions are called interference fringes, the constructive and destructive interference of waves.
If we lower the intensity of light (or the flux of electrons), we should be able to see each photon (electron) strike the screen. Each photon (electron) makes a dot on the screen, but where is the interference pattern?
The interference pattern is still there, it simply takes some time for enough photons, or electrons, to strike the screen to build up a recognizable pattern. Interference, a wave phenomenon, is still occurring even if we only let the photons, or electrons, through one at a time. So what are the individual particles interfering with? Apparently, themselves!
One can now demonstrate this phenomenon in an introductory class.
With Two-Slit Interference, One Photon at a Time, TeachSpin has built an apparatus that allows students to encounter wave-particle duality with photons, the quanta of light. With this instrument, students perform the seminal two-slit interference experiment with light, even at the limit of light intensities so low that they can record the arrival of individual photons at the detector. That raises the apparent paradox which has motivated the concept of duality: in the very interference experiment which makes possible the measurement of wavelength, one observes the arrival of light energy in particle-like quanta, individual photon events. How is it possible for light to propagate as if it were a wave and yet to be detected as if it were a particle? This paradox is the central theme in Richard Feynman's introduction to the fundamentals of quantum mechanics:
"We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by explaining how it works . . . In telling you how it works we will have told you about the basic peculiarities of all quantum mechanics."
It is the purpose of this apparatus to make the phenomenon of light interference as concrete as possible, and to give students the hands-on familiarity which will allow them to confront duality in precise and operational terms. When they have finished, students might not fully understand the mechanism of duality - Feynman asserts that nobody really does - but they will certainly have had direct experience of the phenomenon itself.