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Double slit experiment -WHAt is it it showing or proving

  1. Aug 4, 2008 #1
    Double slit experiment --WHAt is it it showing or proving

    Im just not getting the general concept of what this experiment proves.
    Please explain in very simple language --thanks!
     
  2. jcsd
  3. Aug 4, 2008 #2
    Re: Double slit experiment --WHAt is it it showing or proving

    Particle/wave duality for starters. The electron 'particle' interferes with itself indicating it behaves as a wave.
     
  4. Aug 4, 2008 #3

    G01

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    Re: Double slit experiment --WHAt is it it showing or proving

    The double slit experiment is important because able to provide direct evidence of whether something has wave like properties or not.

    For instance, the fact that light produces the famous double-slit interference pattern from passing through the slits is direct evidence of it's wavelike properties. Only waves will produce such a pattern. This is how Thomas Young found evidence for the wavelike nature of light.

    Along the same lines, the double slit experiment performed with electrons also produces a similar interference pattern. Thus, this is evidence for the fact that electrons, have wave-like properties.
     
  5. Aug 4, 2008 #4
    Re: Double slit experiment --WHAt is it it showing or proving

    I don't mean to push this topic into an undesired direction, however I think an examination of the other side of the concept could shed some... light... on a few questions some of us have tucked away in our minds:

    If the double-slit proved wave-like properties of light.. what was it that proved particle-like properties of light?
     
  6. Aug 5, 2008 #5
    Re: Double slit experiment --WHAt is it it showing or proving

    The photoelectric effect.
     
  7. Aug 5, 2008 #6
    Re: Double slit experiment --WHAt is it it showing or proving

    Doesn't a prism prove the same thing?

    It often seems that the particle/wave question is presented to us layman as an either/or situation, is it a wave, is it a particle, but water and electricty consist of particles that can "wave" so I don't really understand what is the issue at hand.
     
  8. Aug 5, 2008 #7

    Defennder

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    Re: Double slit experiment --WHAt is it it showing or proving

    How does a prism prove particle-wave duality?
     
  9. Aug 5, 2008 #8
    Re: Double slit experiment --WHAt is it it showing or proving

    I doubt the prisim proves anything about the wave-particle duality, but it does prove whether the source of light is purely one frequency or not haha.

    The photoelectric effect, i.e. the jumping of electrons from inner shells to outer shells when shone on by light, proves the particle theory of light because the energy needed to cause an electron to jump say from shell 1 to shell 2, is very specific (quantised), and if light was a wave, light would deliver a continous energy, not in "bursts" which is needed to jump the electron to an outer shell.

    The explanation of what does light actually exist in is photons, which are "particles" of light. And! That these particles are actually "packets of waves". So this provides a way for the wave-particle duality theory can be fathomed.
     
  10. Aug 5, 2008 #9
    Re: Double slit experiment --WHAt is it it showing or proving

    It shows that light can be diffracted and undergo interference, which are characteristic properties of a wave.
     
  11. Aug 5, 2008 #10

    HallsofIvy

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    Re: Double slit experiment --WHAt is it it showing or proving

    No. Water waves are waves in water- water does not itself consist of waves. When we say that light can be considered either a wave or a particle we are not talking about wave of light particles.

    Of course, Quantum Physics says that we can consider anything sufficiently small to have both wave and particle properties.
     
  12. Aug 5, 2008 #11
    Re: Double slit experiment --WHAt is it it showing or proving

    If you supposed that light was a particle-free wave then what would that wave be made of? If it was a wave expressed in the particles of something else like a sound wave then how would light traverse a vacuum?
     
  13. Aug 5, 2008 #12
    Re: Double slit experiment --WHAt is it it showing or proving

    The photo electric effect proves the particle aspect. I'm not sure that the double slit experiment does that or not. The rainbow of color that comes out of a prism is due to the changing the wave lengths of the light entering it.
     
  14. Aug 5, 2008 #13

    Andy Resnick

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    Re: Double slit experiment --WHAt is it it showing or proving

    Young's original experiment was performed to show that light has wave-like properties. In order to do this, he needed to prepare a special state of light, and carefully control the slit geometry.

    The experiment is a very fundamental type of measurement- it measured how spatially coherent the incident wave is (although at the time, the concept of coherence was not known). The experiment was later used to demonstrate that massive particles- electrons originally, but later on atoms and molecules- things that were considered to be point particles (or something very close to that) also possess extended wave-like properties.

    That the experiment is still worth discussing indicates how many fundamental concepts are involved with analyzing the results.
     
  15. Aug 5, 2008 #14

    G01

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    Re: Double slit experiment --WHAt is it it showing or proving

    I'm not sure what you meant here, Landru, but the prism does not change the wavelength of incident light to produce the rainbow.

    A rainbow produced by a prism is due to the fact that the index of refraction of the glass is different for different wavelengths of light. Thus, different wavelengths are bent by different amounts. So, white light entering a prism will broken up into it's component colors to produce the rainbow you see from the prism.
     
  16. Aug 5, 2008 #15
    Re: Double slit experiment --WHAt is it it showing or proving

    I'm just saying a prism wouldn't work if light didn't have wave properties.
     
  17. Aug 5, 2008 #16

    Defennder

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    Re: Double slit experiment --WHAt is it it showing or proving

    Before the Michelson Morley experiment, many people would tell you that EM waves are waves propagating along the ether medium. But no one ever envisaged of the ether as being composed of particles, except perhaps those particle-minded physicists like Newton who subscribed to the Cartesian notion of a mechnanical world.
     
  18. Aug 6, 2008 #17
    Re: Double slit experiment --WHAt is it it showing or proving

    Firstly, Layman speaking here. Possibly a moderately self educated layman who has yet to explore certain fundamental areas of the topic to have a full understanding of it. So please understand when I ask questions that I'm asking for my own edification, rather than to try and appear smarter than I know I am. That said; Thanks to all who are willing to walk with me through this discussion, because these things touch the core of what I have spent a great many of my years trying to comprehend on a practical level.


    I'm not extremely familiar with the relationship between the photoelectric effect and quantum mechanics. I do understand (I think) how it's been described, and how those better versed in the specifics of the topic try to explain it to the layman (i.e. me). However, it comes down to things like this where one doesn't really have the means to experiment and verify touted results personally, and is forced to take literature on a good-faith basis. I'm not sure that I'm ready to do that just yet, so that leaves me with many unanswered questions.

    I guess my main issue here is that I don't really understand what it is about the mechanics of the photoelectric effect that can't be described with pure wave theory in some form. For example; Could not a summation of two differing wavelengths provide the precise 'burst' of energy required to jump electrons to an outer shell at a consistent frequency? I'm considering that each waveform would provide continuos energy onto the particle independent of each other, but the summation would create a third sine wave oscillation effect onto the particle that would apply the correct energy at one of the infinite crossections of the wave summation to have the observed effect of shell jumping. Which doesn't even account for the great number of wave combinations that a particle experiences from any particular light source.

    Even narrow-band laser light should emit overlapping wavelengths that would cause the same effect- albeit slightly different variations of the effect as compared to a wider-band emission source.


    I expect that there is a mathematical reason why this exact explanation couldn't be used to describe the photo-electric effect, however I'm just trying to seek out the reasoning behind why pure-wave form photons were ruled out entirely beyond the shadow of a doubt as a possibility.
     
  19. Aug 6, 2008 #18

    Defennder

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    Re: Double slit experiment --WHAt is it it showing or proving

    This is incorrect. The photoelectric effect is about electrons being liberated (emitted from metal surface) and not merely promoted from an inner shell to an outer shell. The theory which makes use of the electrons being promoted from the inner shells to the outer is the one on gas absorption of light.
     
  20. Aug 6, 2008 #19

    Defennder

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    Re: Double slit experiment --WHAt is it it showing or proving

    The experiment showed that the intensity of incident light on the metal made no difference if there was no photoelectric current observed for a particular frequency of the light source. In classical EM theory, we would expect the energy of the incident light wave to be dependent only on the amplitude of the waveform. The photoelectric effect disproved that notion, because it showed that frequency of light mattered as well. But of course this made no sense in the wave theory, whereby frequency has no influence on the energy of the wave.

    As for whether the electrons could "accumulate" energy from incident waves of low frequency before emission, the photoelectric effect showed that it wasn't possible. I remember my physics lectuer once said that it's not impossible to liberate metal surface electrons with lower frequency light source by "accumulating" energy, but the overwhelmingly mode of photo-electric emission is by all-or-nothing energy from the light source, not by steady accumulation.

    The setup you described of using light of different wavelength isn't applicable here because the photoelectric experiment only made use of monochromatic light sources. Otherwise, no conclusion could be drawn from the experiment if light of different colours were shone on the metal.
     
  21. Aug 6, 2008 #20

    Andy Resnick

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    Re: Double slit experiment --WHAt is it it showing or proving

    The Michaelson-Morley experiment had nothing to do with the wave/particle nature of light (or matter). The corpuscular theory of light was held in prominence well into the 1800's: Poisson, in particular, was one of the last hold-outs.

    The aether was assumed to be a continuous field of something that pervaded all the universe.
     
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