![]() His work in the 1960s on long-distance signal transmission using very pure glass fibers and lasers revolutionized telecommunications, enabling innovations such as the Internet. Our understanding of the phenomenon explored in this Science Snack is built on the work of many scientists.īorn in Shanghai and raised in Taiwan and Hong Kong, Sir Charles Kuen Kao was an electrical engineer and physicist who is regarded as the "father of fiber optics" and the "godfather of broadband". This demonstration can also be done by replacing the aquarium and water with a small transparent plastic block, which can be bought at a local plastics supply store. This helps prevent the signal from weakening too rapidly over long distances, or from leaking out when the fiber goes around a curve. Any light that is not aligned parallel to the axis of the fiber hits the wall of the fiber and is reflected (totally!) back inward, since the angle of incidence with which the light hits the wall is much larger than the critical angle (click to enlarge the diagram below). Total internal reflection helps transmit telecommunications data along optical fibers. ![]() The critical angle for water is measured between the beam and a line perpendicular to the surface, and is 49 degrees. This phenomenon is called total internal reflection, because very nearly 100 percent of the beam is reflected, which is better than the very best mirror surfaces. ![]() At some angle, the bending will be so strong that the refracted beam will be directed right along the surface that is, none of it will get out into the air.īeyond that angle (the critical angle), all the light is reflected back into the water, so the reflected beam is as bright as the incident beam. If the light is moving from a material with a low speed of light into a material with a higher speed of light (for example, from water into air), the bending is toward the surface. The farther the beam is from perpendicular when it hits the surface, the more strongly it is bent. The light beam is bent, or refracted, as it passes from one material into the next (click to enlarge the diagrams below). In general, when a beam of light (the incident beam) hits the interface between two transparent materials, such as air and water, part of the beam is reflected and part of it continues through the interface and on into the other material. At this angle, called the critical angle, all the light is reflected back into the water. Also notice that the transmitted beam is bent, or refracted.Įxperiment until you find the angle at which the transmitted beam completely disappears. Notice that the beam reflected into the water grows brighter as the beam transmitted into the air becomes dimmer. Slowly change the angle at which the beam of light hits the surface of the water. Notice that most of the beam leaves the water and only a faint beam is reflected back down into the water (click to enlarge the illustration below). You can also search for the beam and track it with a piece of paper.) (Dust in the air helps you see the refracted beam. ![]() In the aquarium, you may be able to see both the reflected beam, which bounces back into the water, and the refracted beam, which comes out of the water and into the air. Point the beam so that it hits the surface of the water at just about a right angle. (With the Mini Maglite, you can seal the light in a watertight plastic bag and place the light right in the water!) The beam will be more visible if you can dim the room lights. You can shine the beam into the water through the transparent bottom of the aquarium, or in through a side wall. Direct the light beam upward through the water so that it hits the surface of the water from underneath. ![]()
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