Results 1 to 12 of 12

Thread: Polarisation

  1. #1
    Bad address email on file Corey Nicholls's Avatar
    Join Date
    Apr 2001
    Location
    Canberra, ACT, Australia
    Posts
    196

    Confused Polarisation

    I was having a talk to an optometrist the other day about polarising filters and we got into the topic of which direction the filter actually runs in the lens.

    Now, correct me if I am wrong, but I was taught that even though the axial marks on the side of a polarising filter before you cut it run at 180 degrees, the filter itself runs at 90 degrees thereby cutting out the plane reflected light at 180 degrees (eg like in one of those "Nu-Polar" polarising kits that has the fluro tube sitting above a flat piece of glass, and by putting the filter infront of your eyes you take out the reflected light travelling in the 180).

    The Optometrist seemed to think that for some reason, the filter was set at 45 degrees. The problem I see with this theory, is that you could very easily have one set at 45, and the other set at 135, while still cutting the lens with the axial marks at 180 degrees.

    So after all that, if anyone can understand my ramblings, am I correct?

  2. #2
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    Hi Corey,

    Yes, the filter is oriented so that it absorbs light vibrating in the horizontal (180) plane. At a certain angle (Brewster's angle), light reflected off a horizontal surface becomes completely plane-polarized in the horizontal direction.

    Best regards,
    Darryl

  3. #3
    Bad address email on file Corey Nicholls's Avatar
    Join Date
    Apr 2001
    Location
    Canberra, ACT, Australia
    Posts
    196

    Question Brewster's Angle......

    Darryl,

    Can you explain Brewster's angle to me? If you can do it in the most basic of ways so I can pass it onto the Optometrist!

    Thanks.

  4. #4
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    Hi Corey,

    When light is reflected off a surface, such as water, the amount of light reflected depends upon both the angle of incidence and the particular plane that the light is vibrating in. Meaning, light vibrating in the horizontal direction/plane is reflected by a different amount than light vibrating in the vertical direction/plane for most angles. The greater the angle of incidence, the greater the amount of light reflected. If the angle of incidence is 0 deg (i.e., like your line of sight is normal to the surface), the amount of light reflected r for either the horizontal or vertical plane depends upon a simple relationship given by Snell:

    r = [(n' - n)/(n' + n)]^2 = [(n - 1)/(n + 1)]^2 in air

    For water, the amount of light reflected at an angle of 0 deg is around 2%. For hard resin spectacle lenses, it's around 4%.

    If the angle of incidence is 90 deg, 100% of the light is reflected in both the horizontal and vertical planes. In between these two angles, however, light vibrating in the vertical plane is generally reflected less than light vibrating in the horizontal plane. And, at Brewster's angle, no light vibrating in the vertical plane is reflected for non-absorbing materials -- just light vibrating in the horizontal plane. Brewster's angle b is given by:

    b = atan(n'/n) = atan(n') in air

    For water, Brewster's angle is about 53 deg. At this angle, water reflects no light vibrating in the vertical plane and roughly 10% of the light vibrating in the horizontal plane. (Keep in mind that, since the light began vibrating equally in both the horizontal and vertical planes, 10% of the light in the horizontal plane represents = 10% of 50% = 5% of the overall quantity of light.)

    Best regards,
    Darryl

  5. #5
    OptiBoardaholic
    Join Date
    May 2000
    Location
    Sydney, Australia
    Occupation
    Dispensing Optician
    Posts
    238
    Hi Darryl and Corey,
    I've been away for a few days and, as usual, missed out on an interesting post while I was away. A great explanation as usual, Darryl. I think, though, that you are giving Snell credit that is actually due to Fresnel. The formula you quote is known to me as the Fresnel reflection factor.
    There is some justice in Snell getting credit for something a Fenchman has done, since, in France, Snell's law is known as Descartes' Law.
    Corey, it's great that you have conversations like these with your optom (it's also nice to have a few wins).
    Regards
    David

  6. #6
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    Originally posted by David Wilson
    I think, though, that you are giving Snell credit that is actually due to Fresnel. The formula you quote is known to me as the Fresnel reflection factor.
    You are exactly right, David. That was a mistake made in haste on my part. How could we ever forget the man who invented those giant lighthouse lenses, anyway? ;)

    Best regards,
    Darryl

  7. #7
    Optical Educator
    Join Date
    May 2000
    Location
    Tampa, Florida
    Occupation
    Dispensing Optician
    Posts
    1,044
    Here is a nice article on Polarization from Vision Care Product News...

    http://www.visioncareproducts.com/05...polarized.html

    written by Randy Smith.

    (also, I see that Diane has an article there!)

  8. #8
    Bad address email on file Corey Nicholls's Avatar
    Join Date
    Apr 2001
    Location
    Canberra, ACT, Australia
    Posts
    196

    Thumbs up Many Thanks...

    Thanks to all, it was good to have a win in an optical discussion with one of the bosses! The information given was gladly passed on to her.

    Thanks again.

  9. #9
    Banned
    Join Date
    Jun 2000
    Location
    Only City in the World built over a Volcano
    Occupation
    Dispensing Optician
    Posts
    12,996
    Shouldn't this be spelled with a "z"?

  10. #10
    OptiBoardaholic
    Join Date
    May 2000
    Location
    Sydney, Australia
    Occupation
    Dispensing Optician
    Posts
    238

    Smilie

    Not in Australia, Chip. Corey has used the correct spelling for Australia (the British spelling).
    Regards
    David

  11. #11
    Forever Liz's Dad Steve Machol's Avatar
    Join Date
    Apr 2000
    Location
    Back in AZ
    Occupation
    Other Eyecare-Related Field
    Posts
    10,252
    Originally posted by David Wilson
    Not in Australia, Chip. Corey has used the correct spelling for Australia (the British spelling).
    Regards
    David
    There's that "British' thing again. Are they ever going to learn how to speak and spell proper English?!

    :D :bbg: :D


    OptiBoard Administrator
    ----
    OptiBoard has been proudly serving the Eyecare Community since 1995.

  12. #12
    OptiBoardaholic
    Join Date
    May 2000
    Location
    Sydney, Australia
    Occupation
    Dispensing Optician
    Posts
    238

    Wink

    Funny that you should say that, Steve! Australia, it seems, is gradually becominbg 'Americanised', partly, I suspect, through the efforts of Mr Gates. For example, we now tend to spell program your way rather than programme, the English way. But, we still cling on to the u in colour, neighbour, labour etc...
    Regards
    David

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •