Double Slit Experiment

5 Key excerpts on "Double Slit Experiment"

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  eBook - ePub

  Scientific Challenges to Common Sense Philosophy

  • Rik Peels, Jeroen de Ridder, René van Woudenberg, Rik Peels, Jeroen de Ridder, René van Woudenberg(Authors)
  • 2020(Publication Date)
  • Routledge

    (Publisher)

  2 . b) A trace is left by the light in the interferometer tuned as in a).

  Formally, classical physics places no constraints on interactions except that they be local. It is assumed that these local interactions never vanish completely and that the electromagnetic wave leaves some trace, for example, heating of the air in the arms of the interferometer. Any such trace can in principle be detected, so the interferometer tuned as in Figure 3.1a leaves an observable trace of the same form; see Figure 3.1b .

  3 Common Sense Explanation in Quantum Physics

  In 1986 Grangier et al. showed that the MZI exhibits interference when we send single photons through the interferometer. MZI interference is also routinely observed with neutrons (Rauch, Treimer, and Bonse, 1974). But even before that, the Double Slit Experiment with attenuated light, where single photons would one by one build up an interference pattern on the detection screen, led to the radical picture of quantum theory: in some sense particles are waves!

  Let us first discuss a quantum particle and a single beamsplitter; see Figure 3.2 . We observe that sometimes the particle passes undisturbed and sometimes it bounces from the beamsplitter as if the beamsplitter was a mirror. The difficulty is that in standard quantum theory, there is no equation describing the motion of the particle to one direction or the other. The equation we have is the wave equation (the so-called Schrödinger equation), which splits the incoming wave to two waves, one reflected and one undisturbed, both propagating on a straight line; see Figure 3.2a .

  Figure 3.2

  A particle source, beam splitter, and two particle detectors. a) The quantum wave of the particle. b) The trace of particles sent through the beamsplitter, found by measuring the local environment of the two arms of the interferometer as a large ensemble of particles are sent through. c) The trace of a postselected ensemble of particles sent to the beamsplitter and detected by D1 . Particles that reach D1 do not leave a trace on the path to D2

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  The Logic of Scientific Discovery

  • Karl Popper(Author)
  • 2005(Publication Date)
  • Routledge

    (Publisher)

  cf.section 76)*1

  The imaginary experiment described below under (a) is intended to refute my assertion that arbitrarily exact simultaneous (non-predictive) measurements of the position and momentum of a particle are compatible with the quantum theory.

  (a) Let A be a radiating atom, and let light from it fall on a screen S after passing through two slits, Sl1 and Sl2 . According to Heisenberg we can in this case measure exactly either the position of A or the momentum of the radiation (but not both). If we measure the position exactly (an operation that ‘blurs’ or ‘smears’ the momentum) then we can assume that light is emitted from A in spherical waves. But if we measure the momentum exactly, for example by measuring the recoils due to the emission of photons (thereby ‘blurring’ or ‘smearing’ the position), then we are able to calculate the exact direction and the momentum of the emitted photons. In this case we shall have to regard the radiation as corpuscular (‘needle-radiation’). Thus to the two measuring operations there correspond two different kinds of radiation, so that we obtain two different experimental results. For if we measure the position exactly we obtain an interference-pattern on the screen: a point-like source of light—and one whose position can be exactly measured is point-like—emits coherent light. If on the other hand we measure the momentum exactly, we get no interference pattern. (Flashes of light, or scintillations, without interference pattern, appear on the screen after the photons have passed through the slits, consonantly with the fact that the position is ‘blurred’ or ‘smeared’ and that a non-point-like source of light does not emit coherent light.) If we were to suppose that we could measure both the position and the momentum exactly, then the atom would have to emit, on the one hand, according to the wave theory, continuous spherical waves that would produce interference patterns; and it would have to emit, on the other hand, an incoherent corpuscular beam of photons. (If we were able to calculate the path of each photon we should never get anything like ‘interference’, in view of the fact that photons neither destroy one another nor otherwise interact.) The assumption of exact measurements of position and

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  Introduction to Laser Technology

  • C. Breck Hitz, James J. Ewing, Jeff Hecht(Authors)
  • 2012(Publication Date)
  • Wiley-IEEE Press

    (Publisher)

  Thus, Einstein’s photons explained not only the photoelectric effect but also other experiments that were conducted later that defied explanation from the wave theory. But what about experiments like Young’s double-slit experiment, which absolutely cannot be explained unless light behaves as a wave? How was it possible to resolve the seemingly hopeless contradiction?

  The science of quantum mechanics developed during the early years of the twentieth century to explain this and other contradictions in classical physics. Quantum mechanics predicts that when nature is operating on a very tiny scale (an atomic scale or smaller), it behaves much differently than it does on a normal, “people-sized” scale, so intuition has to be reeducated to be reliable on an atomic scale.

  As a result of quantum mechanics, physicists now believe that the dual nature of light is not a contradiction. In fact, quantum mechanics predicts that particles also have a wavelike property, and experiments have proven that this property exists. By reeducating their intuitions to deal reliably with events on an atomic scale, physicists have found that the duality of light is not a contradiction of nature but a manifestation of nature’s extraordinary complexity.

  If a laser produces a 1 ns, 1 J pulse of light whose wavelength is 1.06 μm, there are two ways you can think of that light. As shown in Figure 2.11 , you can think of that pulse as a foot-long undulating electric and magnetic field. The period of the undulation is 1.06 μm, and the wave moves to the right at the speed of light. On the other hand, you could think of the laser pulse as a collection of photons, as shown in Figure 2.12 . All the photons are moving to the right at the speed of light, and each photon has energy E = hf = hc/λ.

  Figure 2.11 A 1 J, 1 ns pulse of 1.06 μm laser light pictured as a wave.

  Figure 2.12 A 1 J, 1 ns pulse of 1.06 μm laser light pictured as photons.

  Either way of thinking of the pulse is correct, provided that you realize neither way tells you exactly what the pulse is. Light is neither a wave nor a particle, but it is often convenient to think of light as one or the other in a particular situation. Sometimes, light can act as both a wave and a particle simultaneously. For example, you could envision illuminating the cathode of a photocell with stripes of light from Young’s experiment. Electrons would still be liberated instantaneously in the photocell, proving the particle-like nature of light despite the stripes, which prove light’s wavelike nature.

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  In Search of Divine Reality

  Science as a Source of Inspiration

  • Lothar Schäfer(Author)
  • 1997(Publication Date)
  • University of Arkansas Press

    (Publisher)

  diffraction.

  FIGURE 7

  The Generation of a Diffraction Pattern by Two Slits: When a barrier with a single slit is struck by a wave train (left side of the Figure) it becomes the source of elementary wavelets which spread out in all directions behind the barrier. Here a planar wave train—one whose wave front is a plane—is shown striking a single slit S0 . When the wavelets emanating at S0 strike a second barrier, but with two slits in it, S1 and S2 , each of the latter becomes a new point source of elementary wavelets which spread out as shown. In this process wavelets sent out from S1 will interfere with those sent out from S2 , constructively in some places—where hills of one get to lie on top of hills of the other. If the waves used in the experiment are lightwaves, bright regions—constructive interference—will be seen on the screen to alternate with dark regions—destructive interference. The pattern of fringes of alternating bright and dark regions is a diffraction pattern.

  FIGURE 8 A typical interference pattern obtained by the diffraction of lightwaves by a system of slits. The intensity distribution in the pattern is a function of the number of slits.

  YOUNG’S DOUBLE-SLIT EXPERIMENT

  At the beginning of the nineteenth century, Thomas Young performed diffraction experiments with light, allowing lightwaves to diffract and interfere after they passed an array of slits. For example, when a light beam encounters a barrier with two slits, each of them becomes the source of elementary wavelets which can be seen to emerge behind the screen, superimposing and interfering. Along certain lines from the center of the slits, the waves are exactly in phase (crests are on top of crests, troughs on top of troughs) and reinforce. Along other lines they cancel because they are out of step in such a way that crests of waves from one slit coincide with troughs of waves coming from the other. In the first case, constructive interference enhances brightness. In the second case, destructive interference leads to darkness. Therefore, on a screen behind the two slits a diffraction pattern is observed, a pattern of fringes in which regions of darkness alternate with brightness. From the way this pattern is engendered it is clear that its intensity distribution—that is, the variation of darkness and brightness—is the result of the interference of waves that originate in both slits, not just in one of them. Specifically, if the number of slits is changed, a different number of wavelets will interfere, and the diffraction pattern is changed.

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  Polarized Light

  • Dennis H. Goldstein(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  In the previous section, we saw that the developments in mechanics in the eighteenth century led to the mathematical formulation of the wave equation and the concept of energy. Around the year 1800, Thomas Young performed a simple, but remarkable, optical experiment known as the two-pinhole interference experiment. He showed that this experiment could be understood in terms of waves; the experiment gave the first clear-cut support for the wave theory of light. In order to understand the pattern that he observed, he adopted the ideas developed in mechanics and applied them to optics, an extremely novel and radical approach. Until the advent of Young’s work, very little progress had been made in optics since the researches of Newton (the corpuscular theory of light) and Huygens (the wave theory of light). The simple fact was that by the year 1800, aside from Snell’s Law of Refraction and the few things learned about polarization, there was no theoretical basis on which to proceed. Young’s work provided the first critical step in the development and acceptance of the wave theory of light.

  The experiment carried out by Young is shown in Figure 3.3 . A source of light, σ, is placed behind two pinholes s 1 and s 2 , equidistant from σ. The pinholes then act as secondary monochro-matic sources that are in phase, and the light waves from them are superposed on the screen Σ, and observed at an arbitrary point P . Remarkably, one does not see a uniform distribution of light on the screen. Instead, a distinct pattern consisting of bright bands alternating with dark bands is observed. In order to explain this behavior, Young assumed that each of the pinholes, s 1 and s 2 , emitted waves of the form

  u 1

  =

  u 01

  sin ⁡

  (

  ω t − k

  l 1

  )

  ,

  ⁢ ( 3.60 )

  u 2

  =

  u 02

  sin ⁡

  (

  ω t − k

  l 2

  )

  ,

  ⁢ ( 3.61 )

  where pinholes s 1 and s 2 are in the source plane A , and are distances l 1 and l 2 from a point P (x ,y ) in the plane of observation Σ. The pattern is observed on the plane Oxy normal to the perpendicular bisector of s 1 s 2 where the x axis is parallel to s 1 s 2 . The separation of the pinholes is d , and a is the distance between the line joining the pinholes and the plane of observation Σ. For the point P (x , y ) on the screen, Figure 3.3

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