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Science Forum Index » Optics Forum » Another polarization question
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| surfer |
 Posted: Fri Dec 28, 2007 5:11 am |
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How do polarizers work - those made in plastic film, or those used in
consumer camera filters, or those in sunglasses?
Why do they only work in a limited wavelength range?
TIA
Mike |
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| Helpful person |
| Posted: Fri Dec 28, 2007 5:22 am |
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Joined: 22 Jun 2004
Posts: 677
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On Dec 28, 10:11 am, surfer <opto1...@hotmail.com> wrote:
Quote: How do polarizers work - those made in plastic film, or those used in
consumer camera filters, or those in sunglasses?
Why do they only work in a limited wavelength range?
TIA
Mike
The film polarizer is made of a polymer that is stretched. This makes
the molecules in the stretched direction longer than in the
unstretched direction. You can imagine this being like a set of
venetion blinds.
If the separation of the blinds is small then light will not be able
to vibrate in that direction. Hence the light passing through it is
polarized. This efficiency is dependent on the blind separation as
compared to the wavelength of light.
Camera and spectacle polarizing filters are made the same way except
their qualiy is much higher.
These brilliant and simple polarizers were invented by Land (polaroid
Corporation). |
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| Helpful person |
| Posted: Fri Dec 28, 2007 5:46 am |
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Joined: 22 Jun 2004
Posts: 677
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On Dec 28, 10:27 am, "Fleetie" <flee...@fleetie.demon.co.uk> wrote:
Quote: So are you saying then that the waves that are vibrating parallel
the lengths of the open "slits" are the ones that get through?
If so, have you looked at the wikipedia article on this?
"Note that the polarization direction is perpendicular to the wires; the
concept that waves "slip through" the gaps between the wires is incorrect."
http://en.wikipedia.org/wiki/Polariser
Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.livejournal.com/userinfo.bml?user=fleetie
The wikipedia article is describing wire polarizers. However, you
have got me thinking more about the mechanism. When I have time I
will research this and post back.
www.richardfisher.com |
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| Helpful person |
| Posted: Fri Dec 28, 2007 5:58 am |
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Joined: 22 Jun 2004
Posts: 677
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Quote: The wikipedia article is describing wire polarizers. However, you
have got me thinking more about the mechanism. When I have time I
will research this and post back.
www.richardfisher.com
I believe that the explanation is as follows:
For polymer polarizers:
Vibrations perpendicular to the "blinds" are absobed by the material
which is largely opaque. In the direction of the "blinds" the "gaps"
in the material let the light through. Hence the polarization that is
passed is parallel to the "blinds".
For wire polarizers:
Vibrations parallel to the wires are attenuated because they create an
alternating electric field in the wires. As the wires are not perfect
conductors the energy will be absorbed. Perpendicular to the wires no
such current can flow. Hence the polarization that is passed is
perpendicular to the wires.
Can someone who knows more about the mechanism than me please either
confirm or refute this simple explanation.
www.richardfisher.com |
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| Fleetie |
| Posted: Fri Dec 28, 2007 11:27 am |
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"Helpful person" <rrllff@yahoo.com> wrote
Quote: If the separation of the blinds is small then light will not be able
to vibrate in that direction. Hence the light passing through it is
polarized. This efficiency is dependent on the blind separation as
compared to the wavelength of light.
So are you saying then that the waves that are vibrating parallel
the lengths of the open "slits" are the ones that get through?
If so, have you looked at the wikipedia article on this?
"Note that the polarization direction is perpendicular to the wires; the
concept that waves "slip through" the gaps between the wires is incorrect."
http://en.wikipedia.org/wiki/Polariser
Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.livejournal.com/userinfo.bml?user=fleetie |
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| Fleetie |
| Posted: Fri Dec 28, 2007 3:32 pm |
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Thank-you for your gracious responses.
My reply was based on an assumption that the "iodine" chains
in the plastic film are exactly analogous to the wires in the wire
mesh polarisers, given that the article seems to say that electrons
are free to travel along those chains but not between them.
Beyond that, I'm completely in the dark, so I too would appreciate
clarification from someone who knows.
Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.livejournal.com/userinfo.bml?user=fleetie |
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| Timo A. Nieminen |
| Posted: Fri Dec 28, 2007 4:04 pm |
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On Fri, 28 Dec 2007, Fleetie wrote:
Quote: Thank-you for your gracious responses.
My reply was based on an assumption that the "iodine" chains
in the plastic film are exactly analogous to the wires in the wire
mesh polarisers, given that the article seems to say that electrons
are free to travel along those chains but not between them.
Beyond that, I'm completely in the dark, so I too would appreciate
clarification from someone who knows.
That would be about right. With a classical picture, you need conductivity
to get absorption, and to get polarisation dependent absorption, you need
a direction-dependent conductivity. So analogous to wire grid polarisers,
except that wire grid polarisers might reflect rather than absorb - it's a
question of how high the conductivity is.
A fuller story might need to include some quantum mechanics, since you're
talking about molecules.
A related question is why stretched polymer sheets are birefringent, and
you can answer this in terms of individual molecules possessing "shape
birefringence" or "form birefringence".
--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html |
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| surfer |
| Posted: Sat Dec 29, 2007 2:26 am |
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On Dec 29, 3:38 am, Helmut Wabnig <hwabnig@ .- --- -. DOT .- t> wrote:
Quote: On Fri, 28 Dec 2007 15:27:49 -0000, "Fleetie"
flee...@fleetie.demon.co.uk> wrote:
"Helpful person" <rrl...@yahoo.com> wrote
If the separation of the blinds is small then light will not be able
to vibrate in that direction. Hence the light passing through it is
polarized. This efficiency is dependent on the blind separation as
compared to the wavelength of light.
So are you saying then that the waves that are vibrating parallel
the lengths of the open "slits" are the ones that get through?
If so, have you looked at the wikipedia article on this?
"Note that the polarization direction is perpendicular to the wires; the
concept that waves "slip through" the gaps between the wires is incorrect."
http://en.wikipedia.org/wiki/Polariser
Martin
Wire mesh is made of ectrically conducting material.
A horizontally oriented slot antenna radiates vertically polarized.
Think of the wire mesh as an array of slot antennas.http://hjem.get2net.dk/ole_nykjaer/oz2oe/antenner/10wg.html
Dielectrical antennas made of insulating material
react the other way round.
Think of the plastic molecules as dielectical slots.
w.
All of the interesting discussion so far does not directly address the
spectral properties. The mechanism discussed should work as long as
the slits *wires, etc..) are narrow compared to the wavelength. Why
do polarizers lose extinction ratio as the wavelength grows longer
outside the range? Why would not one polarizer with narrow enough
slits fit all wavelengths longer than a certain starting wavelength?
TIA,
Mike |
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| surfer |
| Posted: Sat Dec 29, 2007 2:54 am |
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Guest
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On Dec 29, 7:26 am, surfer <opto1...@hotmail.com> wrote:
Quote: On Dec 29, 3:38 am, Helmut Wabnig <hwabnig@ .- --- -. DOT .- t> wrote:
On Fri, 28 Dec 2007 15:27:49 -0000, "Fleetie"
flee...@fleetie.demon.co.uk> wrote:
"Helpful person" <rrl...@yahoo.com> wrote
If the separation of the blinds is small then light will not be able
to vibrate in that direction. Hence the light passing through it is
polarized. This efficiency is dependent on the blind separation as
compared to the wavelength of light.
So are you saying then that the waves that are vibrating parallel
the lengths of the open "slits" are the ones that get through?
If so, have you looked at the wikipedia article on this?
"Note that the polarization direction is perpendicular to the wires; the
concept that waves "slip through" the gaps between the wires is incorrect."
http://en.wikipedia.org/wiki/Polariser
Martin
Wire mesh is made of ectrically conducting material.
A horizontally oriented slot antenna radiates vertically polarized.
Think of the wire mesh as an array of slot antennas.http://hjem.get2net.dk/ole_nykjaer/oz2oe/antenner/10wg.html
Dielectrical antennas made of insulating material
react the other way round.
Think of the plastic molecules as dielectical slots.
w.
All of the interesting discussion so far does not directly address the
spectral properties. The mechanism discussed should work as long as
the slits *wires, etc..) are narrow compared to the wavelength. Why
do polarizers lose extinction ratio as the wavelength grows longer
outside the range? Why would not one polarizer with narrow enough
slits fit all wavelengths longer than a certain starting wavelength?
TIA,
Mike
In dielectric films, is it related to polarization of the dielectric
material, inertia or resonant frequency of the dipole, and optical
frequency exciting (polarizing) the dipoles? |
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| Helmut Wabnig |
| Posted: Sat Dec 29, 2007 4:38 am |
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Guest
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On Fri, 28 Dec 2007 15:27:49 -0000, "Fleetie"
<fleetie@fleetie.demon.co.uk> wrote:
Quote: "Helpful person" <rrllff@yahoo.com> wrote
If the separation of the blinds is small then light will not be able
to vibrate in that direction. Hence the light passing through it is
polarized. This efficiency is dependent on the blind separation as
compared to the wavelength of light.
So are you saying then that the waves that are vibrating parallel
the lengths of the open "slits" are the ones that get through?
If so, have you looked at the wikipedia article on this?
"Note that the polarization direction is perpendicular to the wires; the
concept that waves "slip through" the gaps between the wires is incorrect."
http://en.wikipedia.org/wiki/Polariser
Martin
Wire mesh is made of ectrically conducting material.
A horizontally oriented slot antenna radiates vertically polarized.
Think of the wire mesh as an array of slot antennas.
http://hjem.get2net.dk/ole_nykjaer/oz2oe/antenner/10wg.html
Dielectrical antennas made of insulating material
react the other way round.
Think of the plastic molecules as dielectical slots.
w. |
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| Salmon Egg |
| Posted: Sun Dec 30, 2007 1:29 pm |
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Guest
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On 12/28/07 7:11 AM, in article
db43cb60-38e0-41e1-92d7-1fdde783aacc@d21g2000prf.googlegroups.com, "surfer"
<opto1310@hotmail.com> wrote:
Quote: How do polarizers work - those made in plastic film, or those used in
consumer camera filters, or those in sunglasses?
If you wish to understand this at a fundamental basis I suggest the
following program:
1. Start with understanding the index ellipsoid for transparent crystals
(anisotropic media). This explains how polarized light propagates in
crystals. The "active" material in Polaroid material is crystalline.
2. Realize that nothing is changed fundamentally when one or more of the
principal indexes has an imaginary component. (Complex index of refraction)
This imaginary component expresses absorption.
3. Just how these complex indexes vary with axis and frequency is rather
complicated. Tourmaline is an example.
4. Most Polaroid type films have uniaxial microcrystals all aligned to each
other in the same way and held in place by a polymer. There are only two
indexes in a uniaxial crystal, the ordinary index and the extraordinary
index. One an only one of these has a large imaginary component (high
absorption) to the index while the other index is almost pure real. (low
absorption)
5 Thus, one of the polarization states gets absorbed and the other does not
as a beam passes through the film.
Bill |
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| whit3rd |
| Posted: Tue Jan 08, 2008 11:34 am |
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Guest
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On Dec 29 2007, 4:26 am, surfer <opto1...@hotmail.com> wrote:
Quote: All of the interesting discussion so far does not directly address the
spectral properties. The mechanism discussed should work as long as
the slits *wires, etc..) are narrow compared to the wavelength. Why
do polarizers lose extinction ratio as the wavelength grows longer
outside the range?
The absorption of light is not complete-to-extinction, just more
for the targeted component than for its orthogonal. Long
wavelength light can tunnel past such a barrier, but in
principle you can just stack two or three polarizers to
get any extinction desired (at some slight loss of transmitted light).
The wires not only have to be closely spaced compared to
the wavelength, they also have to be stacked to a depth comparable
to the wavelength, to be fully effective. |
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