 Measuring the speed of light shouldn't really be that hard. Light moves with uniform motion through a constant medium so we can use the formula v equals d over t. The tricky part is trying to measure t. Because even if you pick a distance that's really huge, like the distance around the entire earth, the time it takes light to travel that distance is only about 0.13 seconds. So what we need in order for this to work is a very, very good stopwatch. The stopwatch would come from American physicist Albert Mickelson, who between 1878-1926 refined a method that could measure the time taken for light to travel between two points. This is the Mickelson rotating mirror experiment. Array of light from a light source reflects off a rotating eight-sided mirror. The light travels a long distance. In this case, Mickelson had it move about 35 kilometers one way through an old abandoned mine shaft between two mountains in California. The light hits a flat mirror at the end of the shaft, reflects and comes back through another 35 kilometers, then strikes the rotating mirror again, and is sent perfectly into the observer's eye. But how can a spinning mirror be a stopwatch? The trick is that the frequency of the spinning mirror, or how long it takes to make one full revolution, can be adjusted. Starting from rest, the frequency is slowly increased as light shines on it. The light reflecting off the spinning mirror will only be observed if the ray hits the observer's eye exactly. And this only happens when the spinning wheel comes into the exact same alignment it was in as when it originally was hit by the light. And because it's an eight-sided wheel, this occurs for the first time at exactly one-eighth of a revolution. The genius of the Mickelson experiment is that that spinning mirror's frequency can be used like a timer, as long as we remember that it's going to take the light exactly one-eighth of a revolution of that spinning mirror to travel its 35 kilometers one way and its 35 kilometers back the other. Let's look at an example. Let's say that we get the mirror spinning at a frequency of 525 hertz, or 525 revolutions per second, and at that frequency the observer can see the light. We can convert the frequency into a time measurement by using the formula period equals one over frequency. The T in this formula is period, or the amount of time in seconds needed to make one full revolution of the wheel. Running this calculation, we get a period of about 0.001905 seconds. Now remember, this is the time it takes the spinning mirror to make one full revolution. But in this experiment, we're only concerned with the time it takes to make an eighth of the revolution, as this is the amount of time needed to let the beam of light move 35 kilometers there and 35 kilometers back. So we'll take our period and multiply it by one-eighth, to get a time of 2.3810 x 10 to the negative 4 seconds. This is sort of our there and back time. Finally, we can use the uniform motion formula. The total distance will be 70,000 meters. Since the light moved 35 kilometers, or 35,000 meters one way, and 35 kilometers back the other way. And we're going to use a time of 2.3810 x 10 to the negative 4 seconds. Dividing these, we get a speed of light value to be 2.94 x 10 to the 8 meters per second, which is pretty close to the accepted value for the speed of light at 299,792,458 meters per second. Now you know how Mickelson worked out the speed of light, but is that speed correct? How does he know that the speed is the same going forwards and backwards? Check out this veritasium video. He does an amazing job talking about the one-way speed of light versus the two-way speed of light. And after you're finished, you can ask yourself, is it even possible to measure the speed of light?