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Light striking a smooth surface is
reflected from it at an angle equal to and opposite from the angle of
incidence.
Nonmetallic Surfaces
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Some surfaces reflect a very high
percentage of the light that strikes them; others reflect some and
absorb some.
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With dielectric (nonconducting)
materials, a portion of the light that is reflected is polarized. The
plane of polarization is parallel to the reflecting surface, and is
strongest at a single reflectance angle particular to each material.
Polarization diminishes rapidly as the angle changes. The angle of
maximum polarization varies according to the nature of the material
reflecting the light, but for most substances it is between 30 and 40
degrees. Reflected light so polarized can be absorbed by a properly
oriented polarizing filter. Thus, reflections or glare on many surfaces
can be partially or completely eliminated.
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The controlling factor in eliminating
reflections, if the effect is to be maximized with a polarizing filter,
is the necessity of using the reflection angle of the material involved
as the angle of view of the camera. Thus, if the angle of strongest
polarized reflection from the water is 37 degrees, then the camera
angle must also be 37 degrees, or the control of the polarized
reflection will only be partial. Such a camera angle is often not
desirable, and occasionally not even practical. So this mode of control
is not perfect.
Metallic Surfaces
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Although the vast majority of materials
likely to be encountered in general photography have the property of
being able to plane polarize light by reflection, however, metals do
not, for reasons not well understood. Because the reflection of an
un-polarized incident light beam from a smooth metallic surface does
not produce plane polarization, polarizers cannot eliminate such
reflections.
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All is not lost, however. If the incident
beam is already plane polarized when it strikes the metallic surface,
as is the light from various portions of the sky, it will remain plane
polarized after reflection, and the strength of the reflection can be
altered by a polarizing filter in the normal manner. For, even as a
metal surface will not plane polarize a beam by reflection, so it will
not depolarize it.
Opposing Surfaces
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There are occasions when two surfaces,
oriented at an angle to one another, both reflect light toward the
camera lens at approximately the polarizing angle. Because the planes
of polarization of the two reflections are parallel to the reflecting
surfaces, only one at a time can be maximally absorbed by a polarizing
filter. If the angle between the two surfaces is 90 degrees, the filter
will be least effective for one reflection just when it will be most
effective for the other, and vice versa. The best overall effect can
only be partial.
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