Now let’s take a brief look at a quantum interpretation of a two-slit optical interference pattern. We studied these patterns in detail for light in Sections 37-3 and 37-4, and in Section 40-10 we discussed their interpretation in terms of the probability that photons strike various regions of the screen where the pattern is formed. In view of our discussion of the wave properties of electrons, it is natural to ask what happens when we do a two-slit interference experiment with electrons. The answer is: exactly the same thing as we saw in Section 40-10 with photons! We can again use photographic film (Fig. 41-7) or particle counters to trace out the interference pattern, as we did with photons. The principle of complementary, introduced in Section 40-10, again tells us that we cannot simultaneously attempt to apply the wave model and the particle model to describe a single part of this experiment. Thus we cannot predict exactly where in the pattern (a wave phenomenon) any individual electron (a particle) will land. We can’t even ask which slit an individual electron passed through in building up the two-slit interference pattern. If we do determine the slits that the electrons pass through hy scattering photons off them, the electrons recoil, and the two-slit interference pattern is not built up.