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Speed of Light - Speed Of Light Using A Nanosecond...

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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, scissor jack (for height adjustment) Introduction The speed of light is an important quantity because it is the speed at which all electromagnetic waves propagate in a vacuum 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 gain some experience with transmission lines (cables), reflections within them (and how to minimize these), and some sophisticated measuring equipment (a high resoloution digital oscilloscope). 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. For example, Morse code is a basic way to transmit sequences of letters via radio waves. For a radio wave, the quality of the reception is dependent on the wave being properly guided to the receiving circuit – typically the connection is established using an antenna. The radio signal is 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 radio station and other metallic obstructions that can cause interference. Signals traveling through cables may encounter discontinuities (kinks in the wire, imperfections in media) or boundaries (connections) that alter the signals, or even make them bounce back. In your study of mechanical waves, you may have learned that an upward (positive) wave pulse that hits a rigid wall will bounce back in the downward (negative) position; if the wall isn't 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
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wave; this is important because, as you've seen, reflected waves interfere with incident waves and cause weakening or corruption of the original signal. In today's lab, you will use a terminating resistor to minimize such reflections; you will also learn that the impedance (the AC equivalent of resistance) of this terminator and the impedance of the wire used must be matched in order to minimize signal reflection and hence maximize signal transfer. *Faster Than Light? Note that there are situations where motion faster than the speed of light occurs, however, no matter, energy or information actually travels greater than the speed of light. For example, imagine that you are holding a powerful laser pointer aimed at the Moon. If you rotate the laser, the projected spot will travel at a speed given by
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