Measuring the speed of light
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Data Analysis Data Analysis


Determination of flight time t.

Figure 6 shows three thumbnails on the screen of the oscilloscope for different distances s of the moving mirror.

Caution: Note that the signals are adjusted to each individual image to have the same height.

To calculate the time distance Dt (flight time) is sufficient to calculate the "distance" of two common points of the two pulses (Fig. 7).



Figure 6: The pulses on the oscilloscope screen for three different distances s



Figure 7: Measurement of the time.


This step involves calibrating the horizontal time axis. That is why at the top of the screen of the oscilloscope represented a square pulse frequency 10 MHz. The period of such a pulse will be, known, T = 1 / f = 100 ns.

To calculate thus the "distance" of two common points of the two pulses is sufficient to compare the distance to the distance between two points of a square pulse 'are' in a time period, or 100 ns (Figure 8). The comparison can be made with any image editor. One such program, for example, is SalsaJ.


Figure 8: Calibration of the time axis.


Open the program with each one of the pictures of the screen of the oscilloscope you did draw two segments, one between the two pulses in the square and a reference pulse (Figure 8). With the help of the program, measure the length of each segment in pixels (px).

Attention! Try that the two line segments are parallel with the horizontal axis.

Let P1 be the length of the segment corresponding to the square pulse and P2 is the length of the segment corresponding to the distance of the two pulses. Given that the length of P1 corresponds to 100 ns, dividing 100 by P1 shows the reduction of unit length of pixels at a time (ns / px). Multiplying this number by the P2 resulting flight time t.

In this way, complete Table 2 for 5 photos you have taken.



Distance s (m)


Distance 2s (m)



flight time

Dt (ns)


P1 (px)

P2 (px)
































Table 2: Measurements with RCL

Class Report Class Report

 Here the students will write the class report.

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