sol - Speed Of Light Using A Nanosecond Stopwatch Objective...

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Speed Of Light: Using A Nanosecond Stopwatch Objective: To learn how to use a high-resolution (200 Mhz) digital oscilloscope. To determine the speed of light by directly measuring the time interval required for light to traverse a fixed distance on the lab table. Apparatus: Tektronix TDS 2022B Digital Oscilloscope, red laser (battery operated), diode detector (with AC adapter), 50 Ω (thinner) coaxial cable (2), 75 Ω coaxial (thicker, “RG59” imprint) cable (1), coaxial T-connectors (2), 50 Ω cable terminator (2), meter stick, desk lamp, blue jacks (2, for height adjustments) Introduction The speed of light is an important quantity because it is the speed at which all electromagnetic waves (recall your RLC Circuits lab from last semester) propagate and poses a strict limit on how fast matter, energy and information can travel * . In this lab you will not only measure the speed of light pulses but also hopefully gain some experience with transmission lines (cables), reflections within them (and how to minimize these), and some sophisticated measuring equipment. Information in telecommunications is carried in the form of waves or pulses through air or through some waveguide, like a fiber-optic cable or copper wire. Morse code is a basic way to transmit sequences of letters via radio waves; you already have built a tunable circuit to receive voice and music transmissions. There you noticed that the quality of the reception was dependent on the wave being properly guided to the circuit – you probably had to establish a connection from it to the vertical rod on your lab table,
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and from that to the antenna that extended outside the Physics Building. Your signal was also dependent on such things as the ionosphere (the Sun no longer ionizes it at night, so that radio waves bounce off it more effectively then), the location of the station (WCTC is closest) and other metallic obstructions that caused interference. In cables, signals also encounter such discontinuities (kinks in the wire, imperfections in media) or boundaries (connections) that alter them, or make them bounce back. You may already be familiar, from Mechanics, with an upward (positive) wave pulse that hits a rigid wall and bounces back in the downward (negative) position; if the wall wasn't so rigid then the energy is dissipated and the wave pulse is made to die (“terminated”) at the wall. Such terminators reduce or eliminate the reflected wave; this is important because, as you've seen, reflected waves interfere with incident waves and cause weakening or corruption of the original signal. You will use a terminating resistor in this experiment; you will also find that the impedance (the AC equivalent of resistance) of this terminator and the that of wire used must be matched to minimize signal reflection and hence maximize signal transfer. This is similar in Mechanics to tying the end of a thin bungee cord to the end of a thick one, plucking the thin cord and having the wave pulse bounce
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sol - Speed Of Light Using A Nanosecond Stopwatch Objective...

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