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
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
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.
(*) 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.
Dr Philip C D Hobbs
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510