 But first, we'll finish off this segment with a look at the two modern methods for measuring the speed of light. One method uses the time of flight, like Tzu and others did. Here's a typical lab experiment. A high-speed, pulsed laser produces a regular series of very short pulses. The red lines show the beam path. The first mirror directs the beam to the beam splitter. The beam splitter directs half the beam to the first detector and the other half to the second and third mirrors before arriving at the second detector. Both detectors feed the oscilloscope. An oscilloscope can measure up to hundreds of millions of frames per second. The screen of the oscilloscope shows the signal from the first detector on the top trace and that of the second detector on the lower trace. The divisions on the horizontal axis are one nanosecond. The second pulse arrives several nanoseconds after the first because it has traveled a longer distance. Now we move the second mirror 10 centimeters to the left. Because the beam has an out and back path, this reduces the path length by 20 centimeters. The pulse on the trace arrives earlier by 0.67 nanoseconds with respect to the first, giving us the value for the speed of light c equals 30 centimeters per nanosecond. That's one foot per nanosecond. In kilometers that's 300,000 kilometers per second and in miles it's 186,000 miles per second. This method for measuring the speed of light is good, but not good enough for the precision needed for such a key universal constant.