ordinary as blue sky on clear days with scattered clouds
consists of polarized light. Compare the two photographs on the
right. The polarizer is rotated 90 degrees between the two
photos. Notice that the sky is much darker in the top photo than
the bottom. The light from the blue sky is polarized in the
vertical direction. When photographed through a polarizing filter
oriented horizontally, the vertical polarized light from the blue sky
is mostly block and appears dark.
In the lower photo the polarizing filter was rotated so that it allows vertically polarized light to be transmitted. Notice that the sky is much brighter and the clouds are about the same brightness.
Photographers often use polarizing filters to enhance the contrast between clouds and the sky - especially for black and white photography. The polarizing filter technique has a different effect on color photography, however. The color of the sky adds a color contrast between the clouds and sky without the need for darkening the sky. The resulting color in the horizontal polarizer image is rather artificial, somewhat more purple. However, no filter will eliminate the distracting power line from the middle of the photos....
Photos by Donald Collins