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Homework Help: Young's Double Slit Experiment questions

  1. Jan 15, 2008 #1
    I am stuck on a question regarding Young's Double Slit Experiment.

    The two beams in Young's double slit experiment must

    (a) be parallel
    (b) have equal intensity
    (c) come from the same source
    (d) traverse equal distances

    Here are my reasons for my answers. I may be misinterpreting certain parts to this question. Please help correct me on any misunderstandings. There is only one correct answer.

    (a) FALSE - I can eliminate this answer because even if the two beams were shined slightly angled into the slits, only the parallel component of the beam would pass through the slit. We would just end up with less light entering the slit if the entering beams were not parallel.

    (b) TRUE - I guess this answer is assuming that we are using two different sources of light. If we use two different sources and they are not the same intensity then the interference patterns might not appear correctly.

    (c) FALSE - We could use two separate lasers. The light emitted from both lasers will be coherent. Even though the lasers may not have the same phase shift, we will still get an interference pattern as long as they have the same intensity.

    (d) FALSE - The point of this experiment is to cause light to travel different distances which ends up causing a relative phase shift. This produces the cancellation of the light and creates the interference pattern.

  2. jcsd
  3. Jan 15, 2008 #2


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    So thats the equivalent of saying that only the parallel part is used. ie you are actually using parallel beams.

    You will still get an inteference pattern but it won't go down to zero, no perfectly black lines. This is called the visibility, the ratio (vmax-vmin)/(vmax+vmin)

    No (assuming you don't have a complex phase locked laser pair) only indivual photons have constant phase, two photons from the same laser aren'tcoherent never mind photons from separate lasers.

    To produce an interference effect the beams must be a whole number of wavelengths different. In practice they must be within the coherence length of the laser - this is the distance over which the light from the laser keeps phase. It depends on the wavelength/bandwidth and the type of laser .
  4. Jan 15, 2008 #3
    Thanks for all the help! I think I see that my main problem is that I don't quite understand coherence. Only a single photon has the property of coherence? My textbook says:

    "A laser differs from common light sources in that its atoms emit light in a cooperative manner, thereby making the light coherent."

    I also defines coherence as

    "If two light waves that meet at a point are to interfere perceptibly, the phase difference between them must remain constant with time; that is, the waves must be coherent. When the two coherent waves meet, the resulting intensity may be found by using phasors."

    After reading these descriptions of coherence, I don't understand why two separate lasers cannot be used to carry out the experiment. Is there a web page with more explanations? But, in order for this experiment to work, we MUST use a SINGLE source of coherent light. Correct?
  5. Jan 15, 2008 #4


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    Actually I should have said a single wave packet.
    A laser emits photons that are coherent over a certain period of time or length, this is the coherence length. Photons emitted within this time, or this distance apart are coherent.

    You don't have any control on what phase these coherent photons are, they are random, just all random with the same value! So you can't have a second laser with the same phase as the first.
    Actually a way of making extremely stable lasers is to have two lasers pointing at each other, photons from one will trigger emission in the other with the same phase so the lasers phaselock each other - but really this is just a single light source, it just has two laser cavities.
  6. Jan 16, 2008 #5
    Thanks again for the help mgb_phys
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