Return to Home Page QUESTIONS AND ANSWERS Organized by Topics Select Topic Air Conditioners Airplanes Audio Amplifiers Automobiles Balloons Balls, Birdies, and Frisbees Bicycles Bouncing Balls Cameras Centrifuges and Roller Coasters Clocks Clothing and Insulation Compact Disc Players Computers Electric Motors Electric Power Distribution Electric Power Generation Electronic Air Cleaners Elevators Falling Balls Flashlights Fluorescent Lamps Incandescent Light Bulbs Knives and Steel Lasers Magnetically Levitated Trains Medical Imaging and Radiation Microwave Ovens Nuclear Reactors Nuclear Weapons Plastics Radio Ramps Rockets Seesaws Spring Scales Sunlight Tape Recorders Telescopes and Microscopes Television The Sea and Surfing Thermometers and Thermostats Vacuum Cleaners Violins and Pipe Organs Water Distribution Water Faucets Water, Steam, and Ice Wheels Windows and Glass Wood Stoves Xerographic Copiers Other Topics All Questions & Answers Ask a Question

 Question 735

 How do scientists measure the speed of light? — DZ, Illinois
There are many possible methods for measuring the speed of light, but the classic technique is easiest to describe. In this method, a rapidly spinning mirror is used to direct a beam of light down a long pipe toward a stationary mirror at the end of that pipe. The first mirror is spinning in such a way that the beam it reflects sweeps across the pipe and can only strike the second mirror during that brief moment when the first mirror is perfectly aligned to direct the light down the pipe. A scientist then looks into the spinning mirror to observe the flash of light that returns from the second mirror. Because it takes a small but finite amount of time for the light to travel back and forth through the pipe, the spinning mirror will have turned a little between the moment when it sent the beam of light toward the far mirror and the moment when that beam of light returns to the spinning mirror. By studying the angle at which the reflected beam leaves the spinning mirror and by knowing how quickly the mirror is spinning, the scientist can determine the speed of light.

However, something has changed since those sorts of measurements were done: the speed of light is now a defined constant. It isn't measured any more—it's simply defined to be 299,792,458 meters per second. The second is defined in a similar manner—as 9,192,631,770 periods of a particular microwave emission from the cesium-133 atom. Because of these two definitions, an experiment that "measures the speed of light" is now used to determine the length of the meter.