A linear polarizer is a device which selectively allows the passage of only certain
orientations of plane polarized light. At one orientation it might allow the passage of
only vertically polarized light, while if rotated by 90 degrees it would allow the passage
of only horizontally polarized light. Half way in between, at say 45 degrees rotation, it
would allow passage of only 45 degree plane polarized light.
You might ask what the use of such a device is? Well, most light sources (except for some lasers) put out randomly polarized light, i.e. an equal mixture of every kind of polarization. If objects reflected this light with no change, all a polarizer would do would be to act like a neutral density filter, no matter how it was oriented, but that's not what happens.
Partially reflecting surfaces, such as a window or the surface of a pond, often reflect one, linearly polarized, component of the incoming randomly polarized light much more strongly than the others (this is very important as we will see later, since many SLR mirrors are partial reflectors!). If a linear polarizer is rotated so as to block this reflected linear component, the reflection will be suppressed as clearly demonstrated by the images above. This is one classic use of a linear polarizer, to remove reflections.
Polarizers can also increase the saturation of colors by eliminating some of the light reflected from things like leaves or grass, which can also be slightly polarized. Polarizers are well known for their ability to darken the sky (especially around 90 degrees from the position of the sun). This is because the light scattered from that part of the sky is quite strongly polarized by the scattering process. If the polarizer is oriented to block out this polarized component, the sky gets darker. Polarizers can also help in cutting out atmospheric haze, since the source of this haze is also scattered light. Though the scattering process in this case is somewhat different from light scattering by the blue sky, there can still be a small degree of induced linear polarization.
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You might ask what the use of such a device is? Well, most light sources (except for some lasers) put out randomly polarized light, i.e. an equal mixture of every kind of polarization. If objects reflected this light with no change, all a polarizer would do would be to act like a neutral density filter, no matter how it was oriented, but that's not what happens.
Partially reflecting surfaces, such as a window or the surface of a pond, often reflect one, linearly polarized, component of the incoming randomly polarized light much more strongly than the others (this is very important as we will see later, since many SLR mirrors are partial reflectors!). If a linear polarizer is rotated so as to block this reflected linear component, the reflection will be suppressed as clearly demonstrated by the images above. This is one classic use of a linear polarizer, to remove reflections.
Polarizers can also increase the saturation of colors by eliminating some of the light reflected from things like leaves or grass, which can also be slightly polarized. Polarizers are well known for their ability to darken the sky (especially around 90 degrees from the position of the sun). This is because the light scattered from that part of the sky is quite strongly polarized by the scattering process. If the polarizer is oriented to block out this polarized component, the sky gets darker. Polarizers can also help in cutting out atmospheric haze, since the source of this haze is also scattered light. Though the scattering process in this case is somewhat different from light scattering by the blue sky, there can still be a small degree of induced linear polarization.
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Fact: The glare of the sun on the sea (and other surfaces) is highly polarized
!
Indeed, the glare can be almost completely horizontally polarized, depending
on the height of the sun. In addition, all reflections from objects above
water are partially polarized. With polarized sunglasses the sea
appears more transparent! See all the glare on the image to the
left. The rocks below the surface are washed out. Rotate the polarizer
to align its transmission axis vertical and . . . magic: the glare
disappears! Wouldn't this be handy for fishing?
Where is the turtle?
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