polarized sunglasses

[RichD]

How does polarization of light improve a sunglasses
quality? Or the "sunglass experience", so to speak -

That is, I presume the usual definition - the lens
passes light polarized along a single direction.

--
Rich

[Phil Hobbs]

Post by RichD
How does polarization of light improve a sunglasses
quality? Or the "sunglass experience", so to speak -
That is, I presume the usual definition - the lens
passes light polarized along a single direction.
--
Rich

When light reflects obliquely off a dielectric surface, such as water or
the shiny hood of your car, the reflection is partially polarized,
usually with the vertical polarization much weaker than horizontal.
That means that the electric (E) field is vibrating mostly in the
horizontal direction. (I'm assuming that the surface is horizontal and
the light is coming from above, which is the usual situation outdoors.)

So the glints are polarized and the rest of the scene mostly isn't. (*)
Thus polarizing filters that absorb the horizontal polarization
selectively reduce the glints. (Doesn't work for reflection from
metals, of course.)

The reason is interesting. The electric field of a light ray
oscillates, but is directed perpendicular to the propagation direction.
Thus light waves are said to be _transverse_, like the motion of a
guitar string. (Some other waves, such as compression waves in a
Slinky, are longitudinal, and others such as sound in air have no
special direction.)

At an interface between two non-absorbing dielectrics, the reflected and
refracted beams go in different directions, but their fields have to add
up to the same as the incident wave. (It's slightly more complicated,
but this is the gist.) The addition is vectorial, so there's a
difference between horizontal polarization, which stays horizontal, and
vertical, which has to change directions on account of the change in
propagation direction.

It turns out that when the reflected and refracted rays are at 90
degrees to each other, in vertical polarization the reflection goes to
zero and in horizontal polarization it doesn't. The incidence angle
where this happens is called "Brewster's angle" after its discoverer.

For common dielectrics such as water and acrylic paint, Brewster's angle
is around 55 degrees, but the effect is useful over a reasonably wide range.

Cheers

Phil Hobbs

(*) On a very clear day, the light from the blue sky is also polarized,
with the maximum amount of polarization visible when you look at 90
degrees to the sun. The brightness change is easily visible when you
take off the glasses and rotate them in front of your eye.

[RichD]

Post by Phil Hobbs

Post by RichD
How does polarization of light improve a sunglasses
quality? Or the "sunglass experience", so to speak -
That is, I presume the usual definition - the lens
passes light polarized along a single direction.

When light reflects obliquely off a dielectric surface, such as water or
the shiny hood of your car, the reflection is partially polarized,
usually with the vertical polarization much weaker than horizontal.
That means that the electric (E) field is vibrating mostly in the
horizontal direction. (I'm assuming that the surface is horizontal and
the light is coming from above, which is the usual situation outdoors.)
So the glints are polarized and the rest of the scene mostly isn't. (*)
Thus polarizing filters that absorb the horizontal polarization
selectively reduce the glints.

So this type of filter removes the bit we commonly call glare.

But there's no free lunch. So what's the loss or cost?

Post by Phil Hobbs
The reason is interesting. The electric field of a light ray
oscillates, but is directed perpendicular to the propagation direction.
At an interface between two non-absorbing dielectrics, the reflected and
refracted beams go in different directions, but their fields have to add
up to the same as the incident wave. The addition is vectorial, so
there's a difference between horizontal polarization, which stays horizontal, and
vertical, which has to change directions on account of the change in
propagation direction.
It turns out that when the reflected and refracted rays are at 90
degrees to each other, in vertical polarization the reflection goes to
zero and in horizontal polarization it doesn't. The incidence angle
where this happens is called "Brewster's angle" after its discoverer.

I'm familiar with Brewster angle, but unclear how it affects
sunglass performance, i.e. the subjective experience.

--
Rich

[Phil Hobbs]

Post by RichD

Post by Phil Hobbs

Post by RichD
How does polarization of light improve a sunglasses
quality? Or the "sunglass experience", so to speak -
That is, I presume the usual definition - the lens
passes light polarized along a single direction.

When light reflects obliquely off a dielectric surface, such as water or
the shiny hood of your car, the reflection is partially polarized,
usually with the vertical polarization much weaker than horizontal.
That means that the electric (E) field is vibrating mostly in the
horizontal direction. (I'm assuming that the surface is horizontal and
the light is coming from above, which is the usual situation outdoors.)
So the glints are polarized and the rest of the scene mostly isn't. (*)
Thus polarizing filters that absorb the horizontal polarization
selectively reduce the glints.

So this type of filter removes the bit we commonly call glare.
But there's no free lunch. So what's the loss or cost?

Post by Phil Hobbs
The reason is interesting. The electric field of a light ray
oscillates, but is directed perpendicular to the propagation direction.
At an interface between two non-absorbing dielectrics, the reflected and
refracted beams go in different directions, but their fields have to add
up to the same as the incident wave. The addition is vectorial, so
there's a difference between horizontal polarization, which stays horizontal, and
vertical, which has to change directions on account of the change in
propagation direction.
It turns out that when the reflected and refracted rays are at 90
degrees to each other, in vertical polarization the reflection goes to
zero and in horizontal polarization it doesn't. The incidence angle
where this happens is called "Brewster's angle" after its discoverer.

I'm familiar with Brewster angle, but unclear how it affects
sunglass performance, i.e. the subjective experience.
--
Rich

As I said, the glare from dielectric reflections is predominantly
horizontally-polarized. Other light from an outdoor scene is nearly all
unpolarized, i.e. 50% horizontal and 50% vertical, so by getting rid of
the horizontally-polarized light you cut down the glare by a lot while
attenuating the scene only a little. (Normally you want sunglasses to
be darker than that anyway.)

LCD displays may look a bit peculiar, of course.

Cheers

Phil Hobbs

[whit3rd]

Post by RichD

Post by RichD
How does polarization of light improve a sunglasses
quality?

I'm familiar with Brewster angle, but unclear how it affects
sunglass performance, i.e. the subjective experience.

When light hits paint, part of the light reflects, and part penetrates into the
body of the paint and interacts with pigment particles.
The 'reflects' part is white light, and the interacting light is colored by
the pigment.

So, colors of many objects become exceptionally vivid when viewed through
a polarizing system (depending on the incident light source and aim considerations
of the polarizing axis). Take a few snapshots with a polarizer, of random items (fabric, paint, glazed
pottery) that have part-transparent composition. Even oil-finished wood grain just seems to
pop with the right polarizer setting.