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QED diffraction grating

  1. Dec 23, 2003 #1

    I have purchased, read, and understood the concepts in Feynman's book QED: The Strange Theory of Light and Matter. I would like to reproduce the diffraction grating experiment (on pg. 50 for those who have it) for a physics presentation at school in order to prove that light doesnt always follow the "law" of reflection.

    Is there a simple and inexpensive way of doing this?

  2. jcsd
  3. Dec 24, 2003 #2
    I haven't got the book to hand but if you want a cheap reflective defraction grating, use the back of a CD. The reason they look multicoloured when white light reflects off of them is due to the different colours having different wavelengths and then cancelling out, or adding to each other. (I believe Feynman describes the effect in QED with bubbles, or oil floating on water) The CD surface has minute pits all over its surface of a size close to the wavelength of the incident light.
    Use a laser to get a diffraction pattern.

    Alternatively, you can make a diffraction grating using photography. Take a high quality photograph (using a stand for the camera) of equally spaced, thin black lines on white card. Use the negative as a grating - it does work but getting the dimensions right might take a bit of fiddling with.

    Hope this helps
  4. Dec 24, 2003 #3

    Chi Meson

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    The CD is indeed the perfect reflective diffraction object; and they are free!

    I have one additional comment. Make sure the students understand that the pattern you see (several dots equally spaced in a line) is not the result of "sloped" sides of a "groove" in the CD's surface. THe situation is exactly as described in Feyman's book: a reflective surface that has regular strips cut out of it (these are the "pits" of the CD).

    You can use several free CD's that come with computers or that you get in the mail and show that the closeness of lines are different.
  5. Dec 24, 2003 #4


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    You can also come by relatively inexpensive diffraction gratings in 'X-Ray' or 'Rainbow' glasses. No guarantees about quality though.
  6. Dec 25, 2003 #5

    Thanks for the great ideas guys. I don't have a laser at the moment but i'll try the experiment out when i get one. Merry x-mas!
  7. Jan 8, 2004 #6

    Okay! My partner got a laser and we tried the experiment yesterday. When we point the laser beam at a certain angle to the cd, we see 3-4 dots that are not equally spaced form a gentle curve, almost a straight line, on the wall or blackboard in front of us. Is this what we are supposed to be seeing? And how does this infer that light sometimes disobeys the law of reflection?
    Is it showing how light can reflect from the same area into the 4 different dots? If so, wouldn't they be in a line as opposed to a slight curve?

  8. Jan 8, 2004 #7

    Chi Meson

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    Re: Interpretation?

    You should see that the dots describe a straight line. If they are curved, well...

    Set it up like so: aim the laser directly at the CD one or two feet away. Set up the veiwing screen directly behind the laser. YOu should get a central (or zeroth order) dot that follows the expected law of reflection. THere should be other dots, equally spaced, that form a straight line along with the central dot (the central dot should also be the brightest dot.

    The reason that the law of reflection is not holding, is that this is a law for WAVES, and you are now observing light according to its particle behavior. QED explains the rest better tan I can.
  9. Jan 8, 2004 #8


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    The reason that the dots are following a curve is probably due to the gating being circular(disk) not linear.
  10. Jan 12, 2004 #9

    Chi Meson

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    This would no make the set of dots follow a curved path. It might make each dot elongated, but I've never seen this to be very significant. The dots probably seem to follow a curved path becaus the viewing surface is set up at an angle to the plane of diffraction.
  11. Jan 12, 2004 #10


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    Since when does a plane making an angle with the grating cause curved reflections? The lines of the grating are circular and the lasers source in not a point source.

  12. Jan 13, 2004 #11

    Chi Meson

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    This is what was confusing me. I can only assume that it appeared curved from the point of view of the observer, even though we both agree that the dots would not be describing a curved line when viewed from a position perpendicular to the surface. But right now I'm looking at the reflective diffraction pattern of a laser bouncing off a CD, and I see several equally spaced dots forming a line. So I don't know what's going on with Mark's set up.

    WHen viewing the "dots" that are projected on a screen several feet away, they are indeed elongated, but all the dots are still a straight line. I've tried to look at it from several points of view, with varying angles, and I can't even make them appear to suggest a curved line.
    Last edited: Jan 13, 2004
  13. Jan 16, 2004 #12

    Ok, i've gotten the dots in a perfectly straight line now. I believe it was curved before because the laser was not exactly perpendicular to the cd.
    (laser------- |cd)

    I've reread the section in QED and i still cant see how the dots in a line disprove the law of reflection. What Feynman's experiment shows is that the light can reflect off an infinite number of points (on the mirror) to reach a photomultiplier, regardless of whether or not the angle of incidence equals the angle of reflection.

    Now, with the cd, when I shine a laser on it and it produces a line of dots behind it on a wall or screen, how does this show the same thing - that light can bounce off all the cd places between the gaps to reach the same point?

    In the book the light source was shone at an angle to the mirror whereas the laser is shone perpendicular to the cd (0 angle of incidence)- doesn't this affect the reflected rays in that they would be reflected right back to the laser?

    And what do I care what the pattern is on the wall - aren't i concerned with the REFLECTED rays that should be on my side of the cd, not the wall's?

    I'm having trouble understanding this, could anyone explain it to me? Thanks
  14. Jan 22, 2004 #13
    Re: Correction

    You have exactly what QED says you should - a diffraction pattern. If you had a mirror, you would have one reflected spot only - all the other possible reflections cancel out. You have a reflection grating that has small gaps on it - as the light from these gaps ISN'T reflected, then they don't cancel out waves that take other paths - hence the row of dots. The angle of incidence does NOT equal the angle of reflection for the reflected dots other than for the central one.

    Then do it an angle like in the book - I've just set this up and it works lovely.... I can't see your problem.

    ...er.. what? If you are not looking at the reflected rays, what are you looking at?
  15. Jan 22, 2004 #14

    Thanks Adrian, and previous posters.

    I understand the experiment fully now. I did my presentation using the experiment and got a 96% - at my grade eleven level, the teacher was impressed. And one more thing. I noticed that if you shine the laser at the back of the cd on that inner ring around the hole, you get tens of dots (perhaps 30?), in about 3 different parallel lines. Its quite interesting, and i have yet to find out what causes it.

    Anyways, thanks again, and take care.

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