# Modern Physics: Chapter 2 – Double Slit Experiment II

XKCD – Nerd Sniping

In the last chapter I mentioned that the maxima and minima formed with electrons in the double slit experiment were probability maxima/minima. In fact, the intensity of the wave displayed as the result of the experiment is proportional to the probability of finding the electron at a given position.

The wave, in this case, is a function of the position. Let’s call it $\psi (x)$ . Since $I \propto A^2$ , the probability of finding the electron at a given position $x$ is proportional to $|\psi^2(x)|$. $\psi(x)$ is called a wavefunction.

One Electron at a Time

If you send one electron at a time, it gets detected at one point on the screen. However, if you keep detecting the positions of the individual electrons coming per unit time and record them, you seen find the same pattern emerging.

Here’s a nice animation to show you what I mean: LINK

Detectors in Front of Slits

If you had detectors in front of the slits (even if the detectors are very gentle), the interference pattern disappears. Surprisingly, as soon as it gets detected at the slits, the electron starts giving the expected bullet-like particle pattern on the screen instead of the wave pattern: