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
2/28/2010
2/25/2010
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
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
2/22/2010
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
Life Science Library - Light and Vision
2/19/2010
The Science of Light (Part 2)
For centuries people had been noticing another odd but obvious fact: a straight pole stuck in the water at an angle no longer appears straight to an observer. The underwater part seems to slant off in a different direction. In 1621 a Dutch mathematician named Willebrord Snell finally explained this phenomenon. A ray leaving one transparent medium and entering another, he said, is usually split at the surface. One part is reflected, in keeping with Hero’s rule. The other part continues into the second medium. The reason that the stick appears to bend on entry into the second medium is that the light rays bringing its image to the eyes are suddenly bent at that point.
Well, if light rays are bent when they enter the water, does this not dispose of the Greeks’ old idea that light always travels in straight lines? Not at all, said Snell. All it indicates is that light may be deflected somewhat if it enters a new medium. The light was traveling in a straight line through the air; when it reached the water it changed direction, but continued in a different, deflected straight line under the water. Snell tried to measure this deflection in various transparent substances such as air, glass and water. He found that each one varied in the amount that it could bend light. Whereupon he gave a name to the bending itself–refraction. It took him a long time to work out the principle, because it seemed terribly contrary and slippery until he also discovered something else: the angle of incidence of the light also had something to do with the amount of refraction. For example, a ray of light striking water vertically will not bend at all. But if it enters at a slight slant it will bend a little; at a greater slant it will bend a lot. Later researchers were able to give numerical values–called refractive indexes–for the various bending powers of all transparent substances. What Snell never discovered is why light bends.
Life Science Library - Light and Vision
Well, if light rays are bent when they enter the water, does this not dispose of the Greeks’ old idea that light always travels in straight lines? Not at all, said Snell. All it indicates is that light may be deflected somewhat if it enters a new medium. The light was traveling in a straight line through the air; when it reached the water it changed direction, but continued in a different, deflected straight line under the water. Snell tried to measure this deflection in various transparent substances such as air, glass and water. He found that each one varied in the amount that it could bend light. Whereupon he gave a name to the bending itself–refraction. It took him a long time to work out the principle, because it seemed terribly contrary and slippery until he also discovered something else: the angle of incidence of the light also had something to do with the amount of refraction. For example, a ray of light striking water vertically will not bend at all. But if it enters at a slight slant it will bend a little; at a greater slant it will bend a lot. Later researchers were able to give numerical values–called refractive indexes–for the various bending powers of all transparent substances. What Snell never discovered is why light bends.
Life Science Library - Light and Vision
2/17/2010
The Science of Light (Part 1)
The eye responds to light. Every object viewed is seen with light–either the light emitted by the object or light that is reflected from it. But what is light–that mysterious glowing stuff that gushes forth in infinite color and variety from the sun, from light bulbs, from candles, fireflies and fireworks? The question has troubled man for centuries.
The Greeks pondered it and arrived at several conclusions. The Pythagorean school assumed that every visible object emits a steady stream of particles. Aristotle, on the other hand, concluded that light travels in something like waves.
Even though these ideas were gradually modified as man began to study light with more sophisticated equipment some 20 centuries later, the essence of the dispute established by the Greeks remained. One point of view held that light is wavelike in nature, that it is energy gliding through space the way ripples spread across the surface of a still pond. Another faction argued that light must be a flight of particles–like drops of water shooting in a stream from a nozzle. At times, one view prevailed; at times, the other. Only in the first half of the 20th Century was something like a comprehensive answer found. And oddly enough both theories turned out to be right.
To identify anything–solid, liquid, gas or pure energy–scientists study its properties. Using this approach, the ancient Greeks discovered that light travels in straight lines. The second important discovery about light was made by Hero of Alexandria. Experimenting with mirrors, Hero noticed that any beam of light that was angled in toward a mirror would bounce off again at an equal angle. This made possible the following fundamental rule: the angle of incidence (or striking) and the angle of reflection (bouncing off) are always equal. Although many thinkers continued to reflect on the nature of light, progress was slow until early in the 17th Century.
Life Science Library - Light and Vision
The Greeks pondered it and arrived at several conclusions. The Pythagorean school assumed that every visible object emits a steady stream of particles. Aristotle, on the other hand, concluded that light travels in something like waves.
Even though these ideas were gradually modified as man began to study light with more sophisticated equipment some 20 centuries later, the essence of the dispute established by the Greeks remained. One point of view held that light is wavelike in nature, that it is energy gliding through space the way ripples spread across the surface of a still pond. Another faction argued that light must be a flight of particles–like drops of water shooting in a stream from a nozzle. At times, one view prevailed; at times, the other. Only in the first half of the 20th Century was something like a comprehensive answer found. And oddly enough both theories turned out to be right.
To identify anything–solid, liquid, gas or pure energy–scientists study its properties. Using this approach, the ancient Greeks discovered that light travels in straight lines. The second important discovery about light was made by Hero of Alexandria. Experimenting with mirrors, Hero noticed that any beam of light that was angled in toward a mirror would bounce off again at an equal angle. This made possible the following fundamental rule: the angle of incidence (or striking) and the angle of reflection (bouncing off) are always equal. Although many thinkers continued to reflect on the nature of light, progress was slow until early in the 17th Century.
Life Science Library - Light and Vision
2/14/2010
The Eyes of the Night Creatures
Animals that must live in darkness–in the gloom of caves, underground burrows and the night world–have special sight problems. Over millennia of evolution, they have been solved either by the development of oversized eyes or by the sharpening of other senses to supplement weak, ineffective eyes.
The night-hunting owl has eyes so large they cannot turn in their sockets. To follow the path of a scurrying field mouse, the owl swivels its head, and can twist it around to look directly backwards. Certain owls have eyes which are so sensitive that they can detect shapes in light many times dimmer than the minimum required by human eyes.
There are other adaptations that help nocturnal animals to see in the dark. A cat's eyes glow in the beam of approaching headlights because a mirrorlike lining at the rear of each eye reflect the light forward again, giving the receptors in the cat's eye a second chance to register each particle of light. The cat also has an especially large number of cells sensitive to dim light, although it cannot distinguish colors.
Moles, bats and shrews, which live in almost complete darkness, have eyes which have degenerated to such an extent that they are almost sightless. The mole rat is virtually blind; its two rudimentary eyes, no bigger than pinheads, detect only shades of light and dark, and are useful mainly as a danger signal when its burrow is broken into from above.
Life Science Library - Light and Vision
The night-hunting owl has eyes so large they cannot turn in their sockets. To follow the path of a scurrying field mouse, the owl swivels its head, and can twist it around to look directly backwards. Certain owls have eyes which are so sensitive that they can detect shapes in light many times dimmer than the minimum required by human eyes.
There are other adaptations that help nocturnal animals to see in the dark. A cat's eyes glow in the beam of approaching headlights because a mirrorlike lining at the rear of each eye reflect the light forward again, giving the receptors in the cat's eye a second chance to register each particle of light. The cat also has an especially large number of cells sensitive to dim light, although it cannot distinguish colors.
Moles, bats and shrews, which live in almost complete darkness, have eyes which have degenerated to such an extent that they are almost sightless. The mole rat is virtually blind; its two rudimentary eyes, no bigger than pinheads, detect only shades of light and dark, and are useful mainly as a danger signal when its burrow is broken into from above.
Life Science Library - Light and Vision
2/12/2010
Lids and Lashes for Protection
Any sudden movement in front of a human eye causes it to blink. The single eyelid snaps shut in a swift reflex action to protect the eye. In many other types of animals the eye’s protective system is far more elaborate. Some creatures–including most birds–possess three eyelids to help guard their eyes against dust, sand and twigs, as well as to carry cleansing tears across the eye surface. The third eyelid is a semitransparent tissue–the nictitating membrane–that flicks across the eye from the inside to the outside corner. Birds seldom close their upper and lower lids except in sleep, but use their nictitating membranes to blink. Tiny feathers on the inside surfaces of the membrane act as miniature brushes to “dust” the eyes. In ducks and some other waterfowl, the third lid serves another purpose. It houses a clear gogglelike lens which improves the ability of the eye to focus underwater while searching for food, a task for which these birds’ normal vision, adapted for flying, is too farsighted.
Snakes and most fishes have no lids at all; the eyes of both are protected by a tough, glassy coating. When a snake sheds its skin, it sheds the coating too. In fishes, the coating is permanent and the water constantly washes it clean.
Eyelashes of different kinds also protect the eye, shading it from glare and filtering out dust. Long, thin feathers act as lashes for birds; similarly rows of scales circle the lids of lizards. Desert animals, which must protect their eyes from the whirling fury of sandstorms, often grow unusually long lashes. A camel’s lashes may measure as much as four inches.
Life Science Library - Light and Vision
Snakes and most fishes have no lids at all; the eyes of both are protected by a tough, glassy coating. When a snake sheds its skin, it sheds the coating too. In fishes, the coating is permanent and the water constantly washes it clean.
Eyelashes of different kinds also protect the eye, shading it from glare and filtering out dust. Long, thin feathers act as lashes for birds; similarly rows of scales circle the lids of lizards. Desert animals, which must protect their eyes from the whirling fury of sandstorms, often grow unusually long lashes. A camel’s lashes may measure as much as four inches.
Life Science Library - Light and Vision
2/10/2010
Eyes in Strange Places
Many marine creatures can see in all directions at once. Most fishes have eyes placed at opposite sides of their heads, permitting sight through a full sweep of 360 degrees. Other creatures, like the fiddler crab, have eyes on stalks, which can be moved about to extend their visual horizons.
The queen conch, a large, seagoing snail, has it eyes at the ends of two long tentacles. The more familiar land snail also has two hornlike tentacles which are somewhat light-sensitive and supplement the eyes at the base of its tentacles.
The way fishes’ eyes are placed–pointing up, down or sideways–depends on environment. The dlounder is an unusual case, however. It starts life swimming upright in the ordinary way, with an eye on either side of its head. But as the flounder grows, its body becomes round and flat, and it swims along the bottom on what formerly was its side. At the same time, one eye migrates to the other side of the head, so that the full-grown flounder has two upward-looking eyes.
Life Science Library - Light and Vision
The queen conch, a large, seagoing snail, has it eyes at the ends of two long tentacles. The more familiar land snail also has two hornlike tentacles which are somewhat light-sensitive and supplement the eyes at the base of its tentacles.
The way fishes’ eyes are placed–pointing up, down or sideways–depends on environment. The dlounder is an unusual case, however. It starts life swimming upright in the ordinary way, with an eye on either side of its head. But as the flounder grows, its body becomes round and flat, and it swims along the bottom on what formerly was its side. At the same time, one eye migrates to the other side of the head, so that the full-grown flounder has two upward-looking eyes.
Life Science Library - Light and Vision
2/08/2010
A Gallery of Animal Eyes
The most complex and efficient eyes belong to higher animals–especially vertebrates such as fish, birds, and mammals. Like humans, most animals have adjustable lenses and myriads of light-sensitive cells for recording sharp images.
But while human eyes are basically the same except for the color of their irises, the eyes of animals take on an almost infinite range of colors, shapes and sizes. The variations reflect evolutionary adaptations to the animal’s habits and environment.
Night-prowling animals most often have large, circular pupils because their eyes must be able to catch every stray glimmer of light. But animals like the cat, which hunts both night and day, have made special adaptations to allow them to see clearly in both dim and intense light. The cat’s pupil is not around but oval-shaped. At night the oval opens wide, but in strong light it closes to a tight slit. Similarly, the gecko–a lizard which feeds at night–shuts its pupils in daylight but leaves four tiny diamond-shaped holes to admit light.
The green whip snake has an odd pupil shaped like a keyhole, allowing it a sweeping view forward through a horizontal slit and a more direct view to the side through the round portion. Among sea dwellers, the skate has a fringed awning to protect its eyes from strong sunlight. But the oddest adaptation of all belongs to a tropical fish, the Anableps, which swims along the surface with its eyes half in and half out of the water. Each eye has two pupils, one for looking upward into the air and the other for gazing down into the water.
Life Science Library - Light and Vision
But while human eyes are basically the same except for the color of their irises, the eyes of animals take on an almost infinite range of colors, shapes and sizes. The variations reflect evolutionary adaptations to the animal’s habits and environment.
Night-prowling animals most often have large, circular pupils because their eyes must be able to catch every stray glimmer of light. But animals like the cat, which hunts both night and day, have made special adaptations to allow them to see clearly in both dim and intense light. The cat’s pupil is not around but oval-shaped. At night the oval opens wide, but in strong light it closes to a tight slit. Similarly, the gecko–a lizard which feeds at night–shuts its pupils in daylight but leaves four tiny diamond-shaped holes to admit light.
The green whip snake has an odd pupil shaped like a keyhole, allowing it a sweeping view forward through a horizontal slit and a more direct view to the side through the round portion. Among sea dwellers, the skate has a fringed awning to protect its eyes from strong sunlight. But the oddest adaptation of all belongs to a tropical fish, the Anableps, which swims along the surface with its eyes half in and half out of the water. Each eye has two pupils, one for looking upward into the air and the other for gazing down into the water.
Life Science Library - Light and Vision
2/05/2010
Multiple Facets of Compound Eyes
Compound eyes, found in insects and some marine animals, are the type most often observed in nature. The world's oldest eye, preserved as a fosilized rock, has a compound structure much like the eye of a modern horsefly.
Compound eyes comprise hundreds of relatively long tubes bunched together like a handful of soda straws. At the external tip of each tube is a fixed lens that focuses light rays toward a group of light-sensitive cells at the tube's innermost end. Since the tubes fan out slightly, the eye structure is rounded, giving an extremely broad field of vision.
Each tip, or facet, or a compound eye picks up a tiny image of the section of the world in front of it, and transmits this fragment to the brain as a nerve impulse, there to be fused with signals from other facets into an overall mosaic picture. Since facets cannot change focus, compound eyes are unable to form precise images. A wasp cannot tell the difference between a fly on a wall and a nailhead. But compound eyes are extremely efficient at detecting movement. Honeybees always head toward flowers swayed by a light breeze; predactory dragonflies are able to make precise calculations of the speed of smaller insects darting through their visual fields.
The simple eyes of most spiders are like separated facets of a compound eye. They are usually arranged in clusters along the spider's back, in such a way that they can register the movement of images passing in sequence from one eye to the next. Spiders which sit quietly in their webs are not so dependent on their vision as hunting spiders, and therefore can get along with very nearsighted eyes.
Life Science Library - Light and Vision
Compound eyes comprise hundreds of relatively long tubes bunched together like a handful of soda straws. At the external tip of each tube is a fixed lens that focuses light rays toward a group of light-sensitive cells at the tube's innermost end. Since the tubes fan out slightly, the eye structure is rounded, giving an extremely broad field of vision.
Each tip, or facet, or a compound eye picks up a tiny image of the section of the world in front of it, and transmits this fragment to the brain as a nerve impulse, there to be fused with signals from other facets into an overall mosaic picture. Since facets cannot change focus, compound eyes are unable to form precise images. A wasp cannot tell the difference between a fly on a wall and a nailhead. But compound eyes are extremely efficient at detecting movement. Honeybees always head toward flowers swayed by a light breeze; predactory dragonflies are able to make precise calculations of the speed of smaller insects darting through their visual fields.
The simple eyes of most spiders are like separated facets of a compound eye. They are usually arranged in clusters along the spider's back, in such a way that they can register the movement of images passing in sequence from one eye to the next. Spiders which sit quietly in their webs are not so dependent on their vision as hunting spiders, and therefore can get along with very nearsighted eyes.
Life Science Library - Light and Vision
2/03/2010
The Simplest Sight
The most elementary sight organs are not eyes at all, but light-sensitive areas called eyespots which can only detect differences between light and dark. Many simple forms of marine life, like the one-celled Euglena, have eyespots to orient them toward the light, which they use to manufacture energy in much the same way plants do. The eyespots, which contains one or more specks of pigment, also is useful in directing the organism away from the surface during periods of strong sunlight, when the sun's rays could injure it.
During millions of years of evolution, some marine creatures have developed highly specialized and often complex eyespots. The scallop has from 50 to 200 such spots, each of which functions like a simple but unfocused eye.
Unique among microscopic marine life is the pinhead-sized Copilia, which can actually form images by means of its crude visual system. Some scientists think the Copilia may be an evolutionary link in the progression of the organs of vision from eyespots to more complex eyes.
Life Science Library - Light and Vision
During millions of years of evolution, some marine creatures have developed highly specialized and often complex eyespots. The scallop has from 50 to 200 such spots, each of which functions like a simple but unfocused eye.
Unique among microscopic marine life is the pinhead-sized Copilia, which can actually form images by means of its crude visual system. Some scientists think the Copilia may be an evolutionary link in the progression of the organs of vision from eyespots to more complex eyes.
Life Science Library - Light and Vision
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