![]() During the movie, the baffle is moved so as to uncover the second slit: we go from two slits to one. ![]() Throughout the movie in the third screen, the scanning slit is positioned at a minimum in the interference pattern. Further, as we'll see later, a similar conclusion applies to electron behaviour. The square of the amplitude of the sum of the wave amplitudes determines the probability of photon arrival. So the interference term – the (cos 2 φ/2) term determines the probability of photon arrival at any point. How to interpret this pattern? The oscilloscope trace in this experiment is a histogram of photon arrivals as a function of position across the 'screen'. The oscilloscope trace moves with constant speed and, if the micrometer speed were constant, this would mean that position on the oscilloscope screen is linear related to position on the interference pattern.I tried to keep the scanning speed constant by turning equal amounts in a regular rhythm, but the former is hard to achieve with precision.) (For those who have noticed that the spacing between the peaks is not exactly equal: the answer is that the micrometer is moved by hand. The result shows the combination of diffraction and interference: a classic pattern for a two-slit Young's experiment. In this movie, we can both see and hear maxima and minima in the rate of arrival of photons at the PMT. During the movie, the scanning slit is moved across the interference pattern. The baffle covers one slit, so this movie shows the pattern produced by diffraction through a single slit.įor the movie shown below, the baffle has been moved to expose both slits. The PMT output is also amplifed, integrated and input to an oscilloscope, whose screen we see in the foreground movie above. So there is only ever one photon in the apparatus at a time. However, the photons take only two nanoseconds to travel from lamp to PMT. There are as many as several hundred photon arrivals per second, so only milliseconds between between them. So each of the clicks that we hear in this movie records the arrival of a single photon. The PMT output is amplified and input to a loudspeaker. ![]() This process allows a single photon to produce an electric current pulse. That electron is accelerated by a large potential difference so that, when it strikes another electrode, it has sufficient energy to eject many electrons. When a single photon strikes the electrode of the photomultiplier tube, it ejects a single electron. In the foreground is a movie showing the screen of an oscilloscope that displays the rate of photon capture as the scanning slit is moved across the pattern. At right is the box containing the photomultiplier tube. In the midde is the micrometer that positions the baffle to expose one or two slits. The background photo shows the light-tight box, with the lamp end at the left. The scanning slit and the micrometer drive that is used to scan across the interference pattern ![]() The moveable baffle with the micrometer drive that can cover one of the two slits. Looking along the box towards the filter and the lamp. The photos below show some of the components of the aparatus. Thus the PMT can measure the photon arrival rate at different positions on the interference pattern.The whole is in a light-tight box. This baffle is mounted on a micrometer so the slit can be scanned across the interference pattern. Instead of a screen, the patter is formed on another baffle, whch has a small slit, behind which lies a photomultimplier tube (PMT). Turning the micrometer allows the baffle to cover one of the double slits, thus converting from a double to a single slit apparatus. Behind this is a moveable baffle, mounted on a micrometer. This weak beam of light reaches the plate with the double slit. This diverges the beam due to diffraction, and also further reduces the intensity (the number of photons per unit area per unit time). A thin slit allows only a small fraction of its light into the apparatus, and this is filtered so that a further small fraction of photons is admitted.
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