Q7: Why do I keep getting reflections, even if I use a polarizer?
A7: Light which reflects off any surface is polarised to some extent. The
degree of polarisation is related to the angle of incidence of light and
the refractive indices of the two materials. At a certain angle known as
"Brewsters angle", light is 100% polarised. At other angles of incidence
the light is partly polarised.
Brewsters angle is given by
Brewsters angle = arctan( n'/n)
n' is the refractive index of the material giving off the
reflection (eg glass, water).
n is the refractive index of the material through which the
light is incident (eg air).
Refractive indices of common materials (relative to air, n=1)
Material refractive index Brewsters angle
water n=1.333 53
glass n approx 1.5 56
(depends on the glass)
So enough theory,
All the surfaces a photographer wants to control lie in the 50 degree
range. Say you want to take a picture through a glass window. If you have
no filter on you will see a reflection. If you put on a polarizer and take
your picture looking straight through the window the reflection will still
show up. But if you move around so you are looking through the window at
an angle of 50 degrees, the reflected light will be 100% polarized. You
then rotate the polarizing filter on your camera lens until the reflected
image disappears. This is because the direction of polarization can vary
with respect to the camera depending on the angle of incidence of the