Module 5 : MODERN PHYSICS
Lecture 26 : Wave Nature of Particle - the de Broglie Hypothesis
Note that at the minima less energy is received with both slits open than is received with any one slit open, which is strange.
  Let us repeat the experiment with beams of particles, such as electrons, traditionally considered localized objects. The screen consists of an array of detectors which can record arrival of an electron. Experiment is done with a beam of electrons and the number of times a detector records the arrival of electron is noted. This is plotted as a histogram. The distribution of the electrons in the detectors at various points on the screen is what corresponds to the intensity pattern stated above. The plot can be looked upon as a probability distribution curve.
  Let us focus our attention on a detector located at some position $ P$. We close one of the slits and determine the probability of an electron arriving at $ P$. This is done by simply finding out the fraction of the total number of electrons that are emitted by the source $ S$ are detected by the detector at $ P$. When $ S_1$ is open, let this fraction be $ p_1$. Similarly, with $ S_1$ closed but with $ S_2$ open, we determine the corresponding fraction at the same position. If we open both the slits, we would expect the fraction to be $ p=p_1+p_2$, as every electron must pass either through $ S_1$ or through $ S_2$. However, what one finds experimentally is that $ p\ne p_1+p_2$.
   
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