1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Visible rotation

  1. Jan 1, 2006 #1
    Happy new year all- Light can be rotated by a magnetic field (Faraday). Can this rotation be seen by an observer without electronic or computer methods?
     
  2. jcsd
  3. Jan 3, 2006 #2

    Tide

    User Avatar
    Science Advisor
    Homework Helper

    If you rule out electronic means then you rule out any kind of device (even a simple polarizing filter would have to be considered electronic) since you are dealing with electromagnetic fields.
     
  4. Jan 3, 2006 #3
    What if we use magnetic fields?
     
  5. Jan 3, 2006 #4

    Tide

    User Avatar
    Science Advisor
    Homework Helper

    I'm not really sure what you are asking. Are you asking whether we can detect polarized light with the eyes? The answer in that case is "no." Can we see the effect of polarization with the eyes? In this case, the answer is "yes." Reflection of light does depends on its polarization so one could detect changes in polarization by looking for changes in reflectivity, at least for visible light. For wave like RF you have to use "electronic" means such as detection with an antenna whose orientation relative to the polarization determines the amount of signal picked up.
     
  6. Jan 4, 2006 #5
    Let me refine my question: Could an observer see the effects of Faraday rotation with the un-aided eye, without the use of an electric or an electronic apparatus?
     
  7. Jan 4, 2006 #6

    Tide

    User Avatar
    Science Advisor
    Homework Helper

    As far as I know none of the physiological components involved in vision (retina, rods, cones, cornea, lens, etc.) is sensitive to the polarization of light.
     
  8. Jan 4, 2006 #7

    berkeman

    User Avatar

    Staff: Mentor

    As Tide points out, you can use Polaroid sunglasses or grazing reflection to tell the polarization direction of light. Would either of those fit your criteria?
     
  9. Jan 4, 2006 #8

    jtbell

    User Avatar

    Staff: Mentor

    Actually, the retina has a component that is slightly sensitive to polarization. See, for example, http://polarization.com/haidinger/haidinger.html . A Google search for "Haidinger brushes" turns up other references.
     
  10. Jan 5, 2006 #9
    I had read this reference prior to my first post, and while most people seem to have the ability to see these images, not everyone can.
     
  11. Jan 5, 2006 #10
    I'm not interested in determining the polarization of light, but I do want to know if we can see the effects of Faraday rotation itself, without using computers, electromagnetic or electronic methods. Does anyone know if this has been accomplished before or if it is even possible?
     
  12. Jan 5, 2006 #11

    Tide

    User Avatar
    Science Advisor
    Homework Helper

    Interesting read - thanks!
     
  13. Jan 6, 2006 #12
    ...getting back to Faraday rotation, if we could cascade two Faraday rotator devices, wouldn't it be possible to obtain up to 90' of rotation?
    Would four devices in a "series" configuration yield rotation angles up to 180' ?
     
  14. Jan 8, 2006 #13
    Faraday saw it, so I think the answer has to be yes.

    The light in question must first be polarized by passing through a polarizing filter. Faraday used simple reflection off a glass surface as his polarizing filter. He used a "ray of light issuing from an Argand lamp" as his light source. I have no specific idea what an "Argand lamp" is, but I'm sure it was some kind of powerful gas lamp.

    Once the light was polarized he could extinguish the ray with another polarizer at right angles to the first. He set up his electromagnet between these two filters and when it was energized he could suddenly see the image of the flame coming from the second filter. In other words, the magnetic field had rotated the polarized light so it was now oriented to pass through the second filter.

    There is one more thing whose function I don't understand: he also had a piece of a particular kind of glass in between the two polarizing filters, made of "silicated borate of lead". I'm not sure what part this glass played in the phenomenon. I'm also not quite understanding how he had the magnetic field oriented with respect to the ray and the glass, but it has to be oriented in a particular way for this effect to happen.

    In any event, yes, it can be seen with the naked eye.
     
  15. Jan 8, 2006 #14
    Would the introduction of another stage, in-line with the first, contribute to greater degrees of rotation?
    For instance, 45' exiting the first rotator, and 90' exiting the second one, and so on...What would we see? A 180' shift would almost act as a reflection, wouldn't it?
     
  16. Jan 9, 2006 #15
    error corrected

    ...upside down?
     
  17. Jan 10, 2006 #16

    Claude Bile

    User Avatar
    Science Advisor

    The effect is readily visible if a polaroid is placed between the crystal and the observing surface. A 3 mW HeNe laser can be reduced to microwatts with the right applied magnetic field, an easily discernable difference to the naked eye.

    Claude.
     
  18. Jan 13, 2006 #17
    180' rotation effect? Polariod? You mean polarizer? What crystal? What kind of magnetic field is "right"? I don't understand your statement.
     
  19. Jan 15, 2006 #18

    Claude Bile

    User Avatar
    Science Advisor

    Okay, let me clarify my previous post.

    Crystal = the medium that experiences the Faraday effect (Not always a crystal though, but usually).

    Polaroid = A device that polarises, so yes it is a polariser.

    Right magnetic field = The particular magnetic field that gives 90 degree rotation. This value will vary depending on this material one uses.

    Apologies for the confusion.

    Claude.
     
  20. Jan 15, 2006 #19
    Is there some way of rotating the waves more than 90 degrees? If so what would we see?
     
  21. Jan 16, 2006 #20

    Claude Bile

    User Avatar
    Science Advisor

    Of course, the amount of rotation is proportional to the applied magnetic field, and the length of the magneto-optic material. As you rotate the polarisation beyond 90 degrees, the amount of power transmitted through the polaroid will begin to increase again, reaching a maximum at 180 degrees.

    Claude.
     
  22. Jan 16, 2006 #21
    Faraday didn't have a laser. If we were to use a candle flame, and a magnetic field strength great enough to rotate the light 180 degrees, would the exit side of the crystal display the flame up-side down? I think so.
     
  23. Jan 16, 2006 #22

    jtbell

    User Avatar

    Staff: Mentor

    The Faraday effect rotates the plane of polarization of the light (which way the electric field oscillates), not the orientation of an image produced by that light. If you send unpolarized light through a linear polarizer so that it is polarized vertically, then after the Faraday effect the light will still be linearly polarized, but at an angle (i.e. not vertical).
     
  24. Jan 17, 2006 #23
    Pinestone, you need to read in more depth about that polarization is. It has nothing to do with rotating images as a whole.

    In non-technical terms: a photon is like a wavey line drawn on a piece of paper - it is wide in the plane of the paper, but thin at right angles to the paper. It will fit through a small slit if the slit is in the same plane as the paper, which is the plane of the "waves", but it wont fit through in any other orientation. A polarizer offers only thin slits for photons to go through that are all parallel to each other. Think of jail cell bars and a large book: if you hold the book parallel to the bars, it can fit through. If you hold it at right angles to the bars, it won't go through. This is how polarizers "polarize" light: they simply select out all the photons that are oscillating in the same plane by letting them through and block all the ones that aren't.

    What Faraday did with his magnetic field was to rotate the plane of oscillation of the photons. He started out with polarized light, as I mentioned before. He polarized it by reflection, since all reflected light ends up being polarized to some degree. He checked to make sure it was polarized by blocking it with a conventional polarizing filter. What that filter does, is block all the light on a photon by photon basis, because the slits in the material are oriented like the bars in a jail cell that won't let the wide book through. Then, with the magnetic field, he twisted the orientation of all the photons so that they suddenly lined up with the bars, and now they went through!

    The image is not inverted or rotated at all, ever, in any of this, just the individual photons.

    If you've ever worn polaroid sunglasses, or simply looked through a polarizing filter, you can see it doesn't invert or rotate the image. What you should notice most of all is that the glare of any reflected light in the vicinity is much reduced.
     
  25. Jan 18, 2006 #24
    I've experimented with variable polarizing filters before, and have seen their effect. Look at a candle flame through a thin slit in a opaque sheet. You can easly determine the top of the flame from the bottom (color). Didn't Faraday use a flame? When I first learned about his discovery of rotation, I wondered if the entire "length" of the light would rotate. I'm going to try and re-create his apparatus and see for myself. I'll post my observations and conclusions here. Thanks all-
     
    Last edited: Jan 18, 2006
  26. Jun 27, 2006 #25
    Recorded observations here: http://www.magnetostatics.us/photo.htm [Broken] - I'll need your help to resolve the conclusions.
     
    Last edited by a moderator: May 2, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook