cellophane tape and polarization
Tom Ditto
polarizers. He rotated the orientation of the tape on a glass substrate and
placed polarizers on one or either side of the glass. This method produced
unique hues that were correlated to the rotation of the tape.
Can anyone explain the mechanism for this effect and point to literature on
the specifics?
Tom Ditto
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Pieter Kuiper
> I remember seeing a mural made by Stan Vanderbeek that used Scotch tape and
> polarizers. He rotated the orientation of the tape on a glass substrate and
> placed polarizers on one or either side of the glass.
The Scotch tape is between the polarizers.
> Can anyone explain the mechanism for this effect and point to literature on
> the specifics?
The Scotch tape is birerefringent - there is a diference in the index of
refraction between light polarized along the direction of the tape and
perpendicular to it.
You can use "birefringence" to look for images and explanations:
<http://images.google.com/images?hl=en&lr=&q=birefringence>.
Repeating Decimal
on 6/3/02 1:12 PM:
I am not exactly sure what cellophane is. It is some kind of a cellulose
product. Whatever it is, it is birefringent. Whether the birefringednce
arises from oriented molecules possessing asymmetry or merely from stress
birefringence induced in an otherwise isotropic material by the extrusion,
the birefringence is present.
The simplest arrangement is to put a piec or several pieces of cellophane
between crossed polarizers. Without the birefringence, no light gets
through. The birefringence of the celophance causes the polarization of the
light to change.
The amount of birefringence depends upon the orientation of the cellophane
optical axis with the direction of the E-vector of the light. It also
depends upon the thickness of the film and the obliqueness of the light path
through the cellophane and polarizers.
The detailed prediction of colors and other properties requires rather
detailed calculation and a knowledge of crystal optics. This is only a
guidleline to more extended study.
An analytical aid to making the calculations is the Poincare sphere. A
particular polarization state is represented as a point on the sphere. The
effect of retarders (cellophane for example) of various kinds could be
represented as rotations of the sphere about an axis.
To make things more complicated, white light is a broad spectrum. Each
wavelength behaves differently when it passes through. These wavelength
recombine to create new colors.
Bill
Tom Ditto
I had ventured to call the phenomenon bifringence, but I was unable to
understand how polarization caused a color shift. Your instruction and
references have been very useful, and I can now procede to experiment with
the phenomenon. My ambition is to build a variable bandpass color filter
with "bubble gum and scotch tape". It's an amateur astronomy thing.
Tom
Pieter Kuiper
> My ambition is to build a variable bandpass color filter
> with "bubble gum and scotch tape". It's an amateur astronomy thing.
You would get a variable bandstop filter this way. (And not a very good
one, I think.)
John Blandford
> product. Whatever it is, it is birefringent.
It's cellulose acetate, made, I remember from a school visit in 1963, by
dissolving wood pulp in strong caustic soda and then extruding the solution
through a narrow slit into a bath of acid. Or it may be other way round.
The extrusion process must mean that the molecules line up giving it the
anisotropy you need for birefringence. Because the birefringence is
colour-dependent the resulting effects are very beautiful indeed but I think
you need to wipe the adhesive off the Sellotape to see the colours properly.
Colin K.
cleave off a sheet. Should be affordable enough for your project.
Colin K.
"Tom Ditto" <3D(quibble)taconic.net> wrote in message
news:3cfe2...@corp.newsgroups.com...
Repeating Decimal
coli...@yahoo.com wrote on 6/10/02 9:32 PM:
There is no color change, assuming uncolored materials, until light enters
the analyzer. You can try this for yourself using a birefringent material in
a polarascope. Remove the anaylyzer and see if you can get any color. Then
add the analyzer. It is the chromatic absorption change with polarization
state in the analyzer that does the job.
Bill
Daniel P. B. Smith
"Tom Ditto" <3...@taconic.net> wrote:
I dunno if you're going to realize your ambition, but it's a LOT of fun
to get a couple of polarizers and put various transparent things in
between them.
Plastic boxes are neat because you get color fringing effects in the
places where they are stressed and if you twist the plastic you can see
the colors change.
Ditto stretchy plastic as you stretch it.
Not that you're likely to do this but when I was doing biology one of
the most enchanting things I ever saw was a bunch of sea-urchin pluteus
larvae under a microscope between crossed polarizers. These larvae are
almost transparent, but they have a little easel-shape skeleton made of
transparent calcite. Between crossed polarizers everything goes black
EXCEPT the skeletons, which look as if they are glowing and in
color--and as the larvae swim around, the colors change.
(No, I wasn't using any kind of drugs...)
--
Daniel P. B. Smith
dpbs...@theworld.com
Spencer
...
by observing it between crossed polars. :) I confess I wouldn't have
minded exploring further with her, but we mostly kept it to physics
phenomena. Squishing and wiggling the stuff showed a broad range of
colors when back lit with a broad spectrum source.
Then there were the experiments we did that made my long rod "sing".
1/2" aluminum rod and a cloth dampened with acetone. But that's
another story. Happy days. Sigh.
Spencer
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"Daniel P. B. Smith" <dpbs...@bellatlantic.net> wrote in message news:<dpbsmith-454E56...@news.fu-berlin.de>...