When Light Becomes Glare

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

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

Refraction: Bending the Light

Light not only bounces off surfaces, it goes through some of them, often slowing down and changing direction in the process. This directional change, or "bending," is known as refraction, and it occurs at the point where light passes from one medium to another of different density. In the air, light travels at 186,000 miles per second; but water, which is denser than air, slows light down by about one fourth. Glass, which is denser yet, slows it down by a third, and diamond still more. However, for any sort of refraction to take place, the light must strike the new medium at an angle, not head on. The size of this angle determines the amount of bending, a phenomenon illustrated in the photograph above with transparent plastic blocks. Entering from the left, the three light beams hit the first block head on and pass through without bending. But they hit the next block at an angle, causing some of their light to be reflected upward. Most of it, however, enters the block where, slowed by the greater density of the plastic, the beams are bend downward--only to resume their original direction and speed as they leave the block. The third block's two concave surfaces spread the beams apart, but the last block acts as a convex lens and refracts them back together so sharply that they actually cross each other at the right.

The refraction of light produces mirages, rainbows and such bizarre optical effects as the distortion of the girl sitting by the pool at extreme right. It makes a thick-walled glass beer mug look fuller than it really is, and makes the sun appear to set several minutes later than it really does. It also makes it possible to remedy the often faulty refraction in the human eye with corrective eyeglasses.


Life Science Library - Light and Vision

Reflection: Relaying the Image

Although all light can be traced to certain energy sources, like the sun, an electric bulb or a match, most of what actually hits the eye is reflected light--rays that have bounced off various objects and keep right on going. Nearly everything that light strikes reflects a certain amount of its rays, and smooth, shiny surfaces--like the still pool of water at the right--reflect almost as much light as they receive. In fact, it is possible to line a room with mirrors angled in such a way that they will reflect the feeble light of a single candle dozens or even hundreds of times, filling every corner with a brilliance considerably greater than would be possible if the room were covered with black felt, a light-absorbent material which reflect almost nothing.

Light can bounce in many ways, but it always follow a simple rule: the angle of incidence (approach) is equal to the angle of reflection (departure). Despite appearances to the contrary, this rule is being observed by both the flat mirror below, which predictably returns images at equal and opposite angles, and the curved mirror, far right, which sends three identically angled beams leaping outward in three different directions.


Life Science Library - Light and Vision

Rays That Bounce and Bend

Since light is a visual phenomenon, its characteristics are more easily explained with photographs than with words. But in trying to take pictures of light, a peculiar problem presents itself: unless its energy is directed right at the eye or the camera, light is invisible. A man suspended in outer space, with the sun behind him, would see nothing; all would be blackness (save the distant planets and stars) because the energy of the sun would be streaming past him, with nothing to bounce it back to his eye. Standing on the earth's surface, however, he can see trees, houses–even the atmosphere–-all made visible by light bouncing off them and back to his eyes. This phenomenon is exploited in some of the photographs that follow. So that the bouncing and bending paths of different-colored beams of light can be traced, the air has been filled with smoke. The smoke particles help to catch the light and reflect it back toward the camera lens. Similar phenomenon often occur in nature: a beam of sunlight can be seen slanting through a room because it is glancing off dust particles in the air; the shaft of sunlight that are sometimes seen coming down through gaps in clouds are made visible by particles of haze or moisture present in the atmosphere.


Life Science Library - Light and Vision