Light, Reflection and Refraction Of
Whenever a ray of light strikes a surface, at least some of the light is reflected --that is, it bounces off the surface toward a new direction. The surface does not have to be flat or shiny to reflect light; light bouncing off rough surfaces, such as grass or pavement, is reflected in many different directions. This is called diffuse reflection. However, if the surface is both flat and shiny, the light bouncing off it will behave in very specific ways--the reflected rays will be parallel, and they will reflect in an easily calculated angle. This type of reflection is called specular reflection, and it has been used by scientists for centuries to better understand light itself. In specular reflection, the light ray will strike a reflecting surface at some angle (known as the angle of incidence) and bounce away at another angle (the angle of reflection). These angles are measured relative to an imaginary line, called the normal, which is drawn from the point where the ray strikes the reflecting surface and is perpendicular to that surface. As measured from the normal, the angle of reflection will always be equal to the angle of incidence. This is the most important law governing the reflection of light and was first noted by the ancient Greek engineer Hero (first century a.d. ).
If a ray of light is allowed to pass through one transparent medium into another, it will be bent slightly. For example, a long pole stuck in a pool of water will appear to be bent at the water's surface; this is because the light from the pole is bent before it reaches the eye. This bending of light is known as refraction. The degree to which the light is bent depends upon the type of material the light is passing through. The speed of light is often expressed as 186,000 miles (299,200 km) per second; however, this is actually the speed of light in a vacuum (such as space), and is its fastest speed. Light travels more slowly through other materials, such as air or glass, than it does through a vacuum; this is because these materials are comparatively dense, with many molecules in the way. The denser the medium, the slower light will travel through it.
Imagine a ray of light traveling through air and striking a block of glass at a particular angle (the angle of incidence); as the ray enters the denser glass, its speed is reduced, causing it to be bent slightly. The light ray then travels through the glass at this speed for a while until it reaches the edge of the block. Upon exiting the glass into the air the ray will instantly accelerate, once again attaining its original speed; as it enters the air it is bent once again, this time in the opposite direction.
The relationship between these three angles—the angle of incidence and the two angles of refraction--is governed by Snell's Law, as developed by the Dutch physicist Willebrord Snel in 1621. Snell also deduced the indices of refraction (the constant that determines how much a ray of light will be bent) for air, water, glass, and a number of other materials. There is an interesting phenomenon called total internal reflection that juxtaposes both reflection and refraction. If a ray of light travels from a medium with a high index of refraction to one having a lower index (such as moving from water into air) it will be bent away from the normal, toward the first medium. If the angle of incidence is great enough, the ray can be bent so extremely that it will never exit the first medium; in this case, the ray will be reflected off the boundary and back into the medium.
Total internal reflection, first considered little more than a novelty, has found a number of important applications. One of the earliest was in submarine periscopes. Using prisms, light could be internally reflected a number of times and in several directions, allowing a viewer to see around corners and bends.
Probably the most important application of total internal reflection is in the field of fiber optics. An optical fiber is a long, thin tube made of glass or plastic with a very high index of refraction. When light is shone down the tube (usually from a laser) it is reflected and cannot escape through the walls; thus, the light travels along the path of the tube, even around corners and through curves and spirals. Optical fibers have been used to transmit digital light pulses carrying telecommunication signals, such as computer information and telephone calls.
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