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
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