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Homework Help: Physics II Lab - Fizeau's Experiment - Speed of Light

  1. Oct 31, 2009 #1
    1. The problem statement, all variables and given/known data
    I know that the light wave must travel 17.3 km or 17300 meters.
    I found experimentally that the frequency should be 2200 Hz. I picked it from a pool ranging from 0 Hz to 5600 Hz. The goal was to see what frequency was required to get the light to pass correctly through a gap in a wheel.

    I am asked to use the data gathered to calculate "your speed of light."

    2. Relevant equations
    speed of light = wavelength * frequency
    c = (lambda)*f

    3. The attempt at a solution
    I am assuming it is supposed to be around the actual speed of light which is 2.99x10^8 m/s. I am getting a value of 3.806x10^7 m/s by multiplying 17300 m by 2200 Hz.

    Am I using the wrong values? Is there some constant I am overlooking that I need to throw in? Is there another equation?
  2. jcsd
  3. Nov 1, 2009 #2


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    Homework Helper

    You are assuming that the wavelength of the light is 17.3 km... does that make sense?

    I think you might need to describe the context of this question in more detail.
  4. Nov 1, 2009 #3
    Here is the information page from the lab experiment. There is nothing mentioned about wavelengths or frequencies, except what I figured out that the wheel needs to turn at 2200 rev/s.

    Fizeau’s experiment
    A light source sends a beam of light toward a partially silvered mirror. The partially silvered mirror splits the light beam, so part of the beam is reflected perpendicular to its original direction and heads outward (to the right). This outward beam passes through one of the gaps in a rotating toothed wheel, hits a flat mirror, and is reflected back towards the wheel. If the wheel is rotating at the right speed, the reflected beam can pass back through a gap in the wheel to be seen by the observer.

    Although Fizeau’s original experiment used a toothed wheel with 720 gaps, this animation only used 8 gaps. The round-trip distance from the wheel to the flat mirror and back again is 17.3 km, the same as in Fizeau’s setup. Can you use Fizeau’s experiment to determine the speed of light?
  5. Nov 1, 2009 #4


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    Staff: Mentor

    It is not wavelength/frequency problem, it is simple time, speed, distance.
  6. Nov 1, 2009 #5
    Ok, so I have the distance. Where do I get the time and speed? Its a red light, so I know thats a specific wavelength, but will it be similiar to the speed of light at 2.99x10^8 m/s?
  7. Nov 1, 2009 #6
    Thank God for Youtube.

    I found an equation off a video of Fizeau's experiment on Youtube that gave me the equation:

    v = 2*d*N*f

    which is velocity = 2 * distance between mirrors * number of teeth on wheel * frequency

    Plugging all this in, I got 3.04x10^8 which is pretty darn close to the speed of light. At least i'm not off by a magnitude of 10!
  8. Nov 1, 2009 #7


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    Staff: Mentor

    I think it is faster to derive this equation from known data, than to google it. For the light to get through time it spends traveling to mirror and back must be identical to the time needed for next teeth (or rater slot between) to come to the same place. N*f is just a frequency at which the slots appear in the same position - and 1/(N*f) is a period. If light passes through it means it made 2*d in one period, so total distance travelled in 1 sec is 2*d*N*f.
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