Moonlight polarised? Not?

I got to thinking about a moon filter, and thought, why don't I use my
polaroids, as they are very effective at reflected light, speculars, etc.

But they didn't work at all, at any angle, while the moonlight reflected
off the (frozen) birdbath was killed stone dead, like sunlight off wet
roads.

Polaroids are said to work because reflected light is polarised, and are
extremely effective, and the moon's light is reflected, so why does it seem
not to be polarised?

Hoping some of the brains will solve this for me

mike r

On Thu, 26 Feb 2004 22:31:04 +0000 (UTC), in uk.sci.astronomy , mike ring
wrote:

Polaroids are said to work because reflected light is polarised, and are
extremely effective, and the moon's light is reflected, so why does it seem
not to be polarised?

Because the moon isn't transparent ?

AFAIR light reflected off a transparent surface is polarised because one
orientation has a higher angle of total reflection than the other. The moon
isn't transparent, so reflects all the light hitting it, of all
orientations.


--
Mark McIntyre
CLC FAQ http://www.eskimo.com/~scs/C-faq/top.html
CLC readme: http://www.angelfire.com/ms3/bchambless0/welcome_to_clc.html


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mike ring wrote:

I got to thinking about a moon filter, and thought, why don't I use my
polaroids, as they are very effective at reflected light, speculars, etc.

Polaroids are only effective to *polarized* light, not reflected light in
general.

All objects we can see, except those who emits their own light (light
bulbs, stars, fire etc.) we can see because they reflect light.
Artificial light or daylight hits a reflecting surface, some of the light
is absorbed and some are reflected. The wavelength of the reflected part
will decide what colour the object is. (Leafs on a tree looks green
because they absorb all wavelengths except green)

When "ordinary" light, which consists of waves oriented in all different
planes, is reflected of some very "shiny" surface, such as sunlight in
water, somehow the light waves gets oriented in mainly one plane.
Your polaroid sun glasses will filter out waves oriented in this particular
plane and letting waves in all other planes through.

The moon acts like any other object.

Something like that...

"Mark McIntyre" wrote in message
...


On Thu, 26 Feb 2004 22:31:04 +0000 (UTC), in uk.sci.astronomy , mike ring
wrote:

Polaroids are said to work because reflected light is polarised, and are
extremely effective, and the moon's light is reflected, so why does it
seem
not to be polarised?

Because the moon isn't transparent ?

AFAIR light reflected off a transparent surface is polarised because one
orientation has a higher angle of total reflection than the other. The
moon
isn't transparent, so reflects all the light hitting it, of all
orientations.

Nearly but replace transparent with flat.

"Steve Maudsley" wrote in message
...

"Mark McIntyre" wrote in message
...


On Thu, 26 Feb 2004 22:31:04 +0000 (UTC), in uk.sci.astronomy , mike
ring
wrote:

Polaroids are said to work because reflected light is polarised, and
are
extremely effective, and the moon's light is reflected, so why does it
seem
not to be polarised?

Because the moon isn't transparent ?

AFAIR light reflected off a transparent surface is polarised because one
orientation has a higher angle of total reflection than the other. The
moon
isn't transparent, so reflects all the light hitting it, of all
orientations.

Nearly but replace transparent with flat.



Actually what makes the difference is whether the reflection is spectral or
diffuse, a diffuse reflection scatters the light so that not only is the
direction pretty well random, but it is also rotated randomly (not actually
random of course, it depends on the angle of the bit of the surface the
individual light ray strikes) and any previous polarisation lost. With a
surface such as paint, glass, water or similar, the light whose waveform
direction is parallel to the surface is close to 100% reflected, whereas
light at 90° to it is pretty well absorbed, with reflection of angles
between the two varying in proportion. The polarising filter can be
regarded as a sort of molecular slatted blind which transmits light
travelling in the same plane, but stops light in proportion as the angle of
the lights polarisation varies away from this to the 90° at which point, in
theory, the light is absorbed totally. If I remember correctly, light
reflected from a polished metal surface is not polarised, it seems to
require that the surface is transparent, such as the thin varnish layer on
gloss paint, or obviously, water. Interestingly the normal skylight is
polarised to a certain degree, which is why polarising filters are used in
colour photography to darken the sky slightly to improve the appearance, as
well as their use in removing unwanted reflections.

MikeB

"MikeB" wrote in
:



Actually what makes the difference is whether the reflection is
spectral or diffuse, a diffuse reflection scatters the light so that
not only is the direction pretty well random, but it is also rotated
randomly (not actually random of course, it depends on the angle of
the bit of the surface the individual light ray strikes) and any
previous polarisation lost. With a surface such as paint, glass,
water or similar, the light whose waveform direction is parallel to
the surface is close to 100% reflected, whereas light at 90° to it is
pretty well absorbed, with reflection of angles between the two
varying in proportion. The polarising filter can be regarded as a
sort of molecular slatted blind which transmits light travelling in
the same plane, but stops light in proportion as the angle of the
lights polarisation varies away from this to the 90° at which point,
in theory, the light is absorbed totally. If I remember correctly,
light reflected from a polished metal surface is not polarised, it
seems to require that the surface is transparent, such as the thin
varnish layer on gloss paint, or obviously, water. Interestingly the
normal skylight is polarised to a certain degree, which is why
polarising filters are used in colour photography to darken the sky
slightly to improve the appearance, as well as their use in removing
unwanted reflections.

MikeB


Thanks, Mike, I was just about to come back with "Wot acbout shiny
surfaces?", but you read my mind.

Thanks to all for the explanation


Elephant's Child

mike ring wrote:


Thanks, Mike, I was just about to come back with "Wot acbout shiny
surfaces?", but you read my mind.

Sorry about that "shiny" thing. I'm Swedish and you know what swedes are
like

anonymous wrote in
:

mike ring wrote:


Thanks, Mike, I was just about to come back with "Wot acbout shiny
surfaces?", but you read my mind.

Sorry about that "shiny" thing. I'm Swedish and you know what swedes
are like

No, it was very illuminating :-)

I hadn't realised that a clean bright surface was not the same as a
surface which is shiny because of a surface treatment giving a
transparent layer

mike r