Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Harri on 02/07/2017 20:14:22
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I have been reading about the double slit experiment and the surprising results that are witnessed. What I am not sure about is, when the experiment is observed or recorded the results return to what you would expect, just 2 bars of light. How then are the surprising results observed or recorded then?
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You can record the results of the double-slit experiment:
- With your eyes
- on chemical photographic film,
- direct onto a camera CCD (eg a SLR with the lens removed)
- By shining onto a screen, and recording the image from the front or rear
To get a good result, you need:
- A coherent, monochromatic light source, eg a bright laser pointer
- Two slits that are very narrow, and very close together - both distances close to the wavelength of the light being used
- A sensor able to resolve image details that are very fine - similar to the wavelength of the light source.
If the slots are too wide and too far apart, the result will be two bars, ie the interference fringes are too faint and too numerous to be detectable.
See: https://en.wikipedia.org/wiki/Double-slit_experiment
Warning: Do not look straight into a bright laser pointer. Always shine it on a screen and look at the screen.
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In my topic https://www.thenakedscientists.com/forum/index.php?topic=66301.0
I show double slit experiment, which is just a special case of diffraction with four edges obstruction.
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If we assume that light is stream of particles, just like Newton did, the occurence of light and dark bands on the screen is an unexpected result.
If we assume that light is a wave, akin to wave of water surface, then the unexpected result will show when the intensity is extremely low, undetected by naked eye, hence requires very sensitive sensor, such as photomultiplier.
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I guess what I really tried to ask wasn't made clear, and perhaps even now it still doesn't make sense?
I watched this short animation
When scientists try to 'observe' the interference pattern, it resorts back to the original non interference pattern. It's as if it knows it is being observed/watched.
But I was wondering, how do scientists know about the non interference pattern? Surely it is through observation/being watched?
If you watch the video and understand my question, then great. If my question doesn't make sense then ... thanks for reading and for the answers above already given.
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Hi @Harri
I understand what you are saying, which is, I believe, "if making the observation forces the photon to 'choose' a slit and causes the interference pattern to collapse, how do we ever see the interference pattern?"
The answer is that when you observe the pattern you are not observing individual photons but the sum action of many; it's not until you rig the experiment that you are essentially registering single photons that the behaviour breaks down when you try to watch them.
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I guess what I really tried to ask wasn't made clear, and perhaps even now it still doesn't make sense?
I watched this short animation
When scientists try to 'observe' the interference pattern, it resorts back to the original non interference pattern. It's as if it knows it is being observed/watched.
But I was wondering, how do scientists know about the non interference pattern? Surely it is through observation/being watched?
If you watch the video and understand my question, then great. If my question doesn't make sense then ... thanks for reading and for the answers above already given.
A problem in representing an experiment using animation/simulation/model has a merit, which is simplifying the learning process by reducing the number of parameters/factors that influence the result, so students can be more focus on the targeted factor. But it also has its own cost, which sometimes overwhelms the benefits and missing the intended goals, or even showing the wrong results.
For example, I found some learning materials in youtube which shows light dispersion by a prism producing rainbow pattern, but the order of the colors is reversed.
I saw some problems in the video above. First, passing light through a single slit apparatus produces diffraction pattern, instead of a single bright line. Second, it doesn't show the actual method to observe the photon going through each slit. In some papers I've read regarding this experiment, they use polarization filters with different orientation for each slit (e.g. vertical for the left and horizontal for the right, or using clockwise and counter clockwise polarizers). So if we can detect the polarization state of the photon on the screen, we can conclude which slit it has passed through.
But of course we wouldn't get fringes on the screen, because lights with different polarization state don't produce destructive interference.
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ah thanks, now it makes sense. It's difficult to articulate questions when you're a non scientist and not conversant in scientific language.
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ah thanks, now it makes sense. It's difficult to articulate questions when you're a non scientist and not conversant in scientific language.
On the contrary - you asked an excellent and thought-provoking question.