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Not easy question

  1. Jun 27, 2006 #1
    Hi everybody
    How are you? I am a Vietnamese student. I hope to find this forum because I wanna development my lovely subject "physics."

    Here are some question:

    1. Why the sky is blue, and why the ocean is blue, too.
    2. Are there a double rainbow, and how can we see a full circle rainbow
    Why can't you ever find the end of the rainbow.
    3. What is blue moon?
    4. How can you distinguish left hand circularly polarized light and right hand circularly polarized light.

    thanks a lot !
  2. jcsd
  3. Jun 28, 2006 #2


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    Staff: Mentor

  4. Jun 29, 2006 #3
    I had the answers for those questions. Thanks russ_watters ... Your site is very nice!
  5. Jun 29, 2006 #4


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    You can see a nice circular rainbow - with a bit of luck - as you pass out of a cloud in a hot air balloon.
  6. Jul 5, 2006 #5
    For question 4, I'm not sure how much you know about polarisation and optics/photonics, but I'll put in some things........

    If a light wave is traveling in the z-direction, then it's electric field vector, E, lies in the x-y plane. It can hence be resolved into x and y components.

    For linear polarisation, the two components must be exactly in phase or exactly out of phase in time.

    The electric field vector then lies along a constant direction in space, and oscillates sinusoidally in magnitude in time.

    (In the picture, this is the one on the left..... the tip of the E field vector traces along the straight line sinusoidally.)

    For circular polarisation, the two components have equal amplitudes, but are a quarter of a cycle out of phase in time. ie one component reaches it's peak at the same instant that the other passes through zero.

    The resulting electric field vector has a constant magnitude, but it's direction rotates at a constant angular velocity.

    (In the picture, this is the one on the right. The tip of the E field vector traces around the circle. - Don't forget this picture is looking at the wave coming towards you.)

    Our eyes can't distinguish between left and right circularly polarised light (I don't think), but using some optical instruments in the lab, we can find out whether something is left or right polarised.

    The thing you would need is called a quarter wave plate. It is a polariser made out of a material that introduces a phase change of 90 degrees.
    The circularly polarised light would emerge from the plate linearly polarised at 45 degrees to the optical axis, and using another linear polariser we can determine whether the original polarisation was left or right.

    I hope I haven't gone into too much detail and confused you........ and also, I hope I haven't given you too little.

    Have fun!:rofl:

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  7. Jul 5, 2006 #6


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    I've heard that cellophane can make a good quater wave plate, but I've never tried it.

    [add]Google finds
    http://www.sas.org/E-Bulletin/2003-06-20/labNotesAS/body.html [Broken]

    which is unfortunately not free access.
    Last edited by a moderator: May 2, 2017
  8. Jul 5, 2006 #7

    Claude Bile

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    All you need to make a QWP is a birefringent material - i.e. a material where light in one polarisation travels faster than the other. Just about all materials are birefringent to some degree, thus one could make a QWP out of just about any transparent medium they wished.

    Cellophane though is a polymer, and polymers typically have a large birefringence, making them particularly suitable for these types of applications, but there are practical reasons why they aren't used. It could, in principle be done however, though I'm doubtful as to whether it would make a 'good' QWP.

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