A motorist driving over a highway, with the sun in front of him, finds himself squinting in order to see better. A bather at the seashore shades his eyes even on overcast days. In both cases their eyes are being hit by about 10 times as much light as they need, a painful amount. Such concentrations of light, called glare, may result simply from intense sunlight, but more often glare is caused by sunlight reflecting off surfaces like water, snow or sand. Instead of diffusing the light in many directions, these surfaces absorb some of it and reflect the rest—especially the waves which vibrate in a horizontal pattern. Scientists have a word for this selective light-filtering process: polarization.
Polarized glare can be virtually eliminated by sunglasses fitted with polarizing lenses. Such lenses contain a tinted plastic filter with tiny crystals that have been “stretched” into a series of lines, like the slats of a picket fence. The annoying horizontal light is blocked by these lines, but enough of the vertical light vibrations get through the filter so that the sunglass wearer can see to drive or even read. However, if a second set of polarizing lenses is superimposed on the first, so that their lines cross at right angles, so much light is blocked that the wearer is virtually blind. Such an arrangement is often used in space capsules to shield Astronauts from the sun’s glare during their naps.
Life Science Library - Light and Vision
3/11/2010
3/08/2010
Diffraction: Turning a Corner
The trick that light sometimes plays with shadows, giving them bright bends inside their edges, is caused by diffraction—the result of light’s traveling in waves.
If you wiggle a stick in water, a series of waves will flow out in all directions. When they encounter an obstacle, like a plank sticking out of the water, the part of the wave that strikes the very edge of the plank sets up a new series of ripple from that point. Some of the ripples will fan out around the edge—in effect, turning the corner.
That is also what light does. When the waves from a distant pinpoint of light strike an opaque object, they bend around the edges, curving both into the shadow and into the path of other waves from the same light source. Waves bending behind the object create a bright line where the shadow would ordinarily begin. But the waves moving in the opposite direction overlap opposing light waves. Where the crests of the waves meet, they tend to reinforce each other and create bright lines. But where crest meets through they cancel each other, and dark bands result. This overlapping is visible in the picture of a shadow on the opposite page, which has bright edges and distinctive patterns of alternating light and dark bands.
Life Science Library - Light and Vision
If you wiggle a stick in water, a series of waves will flow out in all directions. When they encounter an obstacle, like a plank sticking out of the water, the part of the wave that strikes the very edge of the plank sets up a new series of ripple from that point. Some of the ripples will fan out around the edge—in effect, turning the corner.
That is also what light does. When the waves from a distant pinpoint of light strike an opaque object, they bend around the edges, curving both into the shadow and into the path of other waves from the same light source. Waves bending behind the object create a bright line where the shadow would ordinarily begin. But the waves moving in the opposite direction overlap opposing light waves. Where the crests of the waves meet, they tend to reinforce each other and create bright lines. But where crest meets through they cancel each other, and dark bands result. This overlapping is visible in the picture of a shadow on the opposite page, which has bright edges and distinctive patterns of alternating light and dark bands.
Life Science Library - Light and Vision
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