Beginnings+-+Double+Slit+Experiment


 * Double Slit Experiment**

First, a beam of light is shone on a screen with two narrow slits in it that allow some of the light to pass through to a second screen where an interference pattern is seen. This is a sequence of light and dark bands that are due to the way the separate light waves emerging from the two slits spread out, overlap and merge before hitting the back screen. Where two wave crests or wave troughs meet, they combine together to form a higher crest or lower trough respectively. It is therefore only because light behaves as a wave washing through both slits simultaneously that the interference pattern appears. A similar experiment is carried out using sand. This time the second screen is placed below the one with slits and gravity does the work. As the sand falls onto the first screen, separate piles gradually build up on the lower one beneath the two slits. This is not surprising since each individual grain of sand must pass through one or other of the two slits. (We are not dealing with waves now and there is no interference) The two piles of sand will be of the same height, provided the two slits are of the same size and the sand is poured from a position above their mid-point. Now for the interesting part. The experiment is repeated with atoms. A special apparatus which we would call an atomic gun fires a beam of atoms at a screen with two appropriately narrow slits. On the other side, the second screen is treated with a coating that shows up a tiny bright spot whenever a single atom hits it. Atoms are small entities and should clearly behave in a manner similar to the sand, as opposed to spread-out waves, capable of overlapping both slits at once. When the experiment is ran with bout slits open, we get a spread of light spots on the back screen behind the open slit. This slight spreading of spots may be worrying since this is typical of diffraction, a wave behavior. When the next slit is opened and the experiment is conducted, instead of a pattern observed in the sand experiment, we see an interference pattern of light and dark fringes, just as observed in light. The brightest part of the screen is in the centre where we would not expect many atoms to be able to reach. Despite an atom being a tiny localized particle, it seems that the stream of atoms have somehow conspired to behave in a way similar to a wave. They wash up against the first screen, and those that manage to get through the slits ‘interfere’ with each other’s paths via atomic forces in a way that mimics exactly the pattern that is produced when the peaks and troughs of two waves come together. However, the experiment is redone by firing individual atoms one at a time. That is, only when the flash of light on the back screen signaling the arrival of the atom is observed, do we fire the next atom. There is only one atom travelling through the apparatus at any one time. As the bright spots build up, the same interference pattern is observed. The atom seems to be aware of both slits at the same time. When an atom detector is set up behind one of the slits so it can catch any atom passing through that slit, we find that an atom is registered every now and then. We never catch part of an atom. If the detector is set up in a less intrusive way so as to be able to simply register a signal as an atom passes through that slit on its way to the screen. If an atom is not detected but a hit is recorded on the back screen then it must have gone through the other slit. (Not considering the uncertainty principle) When enough atoms have gone through the slits and you are convinced that half of them went through one slit and half went through the other, you would notice that the interference pattern has disappeared. In its place are two bright patches due to the collection of a pile of atoms behind each slit. The atoms are now behaving like particles. It is only when the atom is being watched that it remains a particle throughout. Clearly the act of observing is crucial. Objects are, in the real world, both particles and waves. We will discuss this further in the chapter.