How did scientists measure the speed of light?

Question

Okay; so we do know what the speed of light is.

But how did scientists manage to measure it in the first place?
Anyone knows the actual way they did it?

Answer 1

The most accurate methods have historically varied depending on the technology of the time. These days, it can be measured directly from time of flight over a known distance using fast photodiodes and electronic recording devices. The greater the distance, the more accurate the estimate

The first measurement, though, required the distance from earth to Jupiter since the clock was basically a sundial. The delay between predicted time verses observed time of an eclipse by Jupiter of one of it's moons was used for the time of flight measurement.

PS The Michaelson-Morely experiment did not measure the speed of light, but rather any small change in it due to its direction. The result was negative.

Answer 2

We do it in Freshman physics lab by using a rapidly rotating mirror (a little piece of silvered glass attached to a power drill). We use the mirror to send a laser beam across the room to another mirror a known distance away. The laser beam comes back to the rotating mirror, which has turned slightly since the beam left it. Since the mirror has turned a little bit, the beam will no longer be reflected back at the laser, but at a slight angle. measure the angle the mirror rotated, the distance the light travelled during that time, and the rotation speed of the mirror. Voila! It's good to 3 sigdigs, which is pretty damn good for a freshman measuring a fundamental constant.

Answer 3

The modern-day (1946 and on) method involves radar. Radar is a form of light (far outside the visible spectrum). You just bounce a radar wave off a mirror far away, and measure how long it takes to get back to you with a very accurate timer. Like how radar gun bounces radar waves off of a car.

There were more complicated methods too, earlier than 1946. Historically, astronomical measurements were initially the most effective at measuring the speed of light. Galileo proposed an experiment similar to the modern-day radar technique (using a lantern and a mirror a mile away), but because they didn't have stopwatches, let alone super-accurate timing devices that could measure nanoseconds, they didn't detect the delay (they were doing it by just looking).

For more information, see the sources.

Answer 4

Look up the Michaelson-Morely experiment. But actually, the speed of light is a defined quantity, not a measured one, so we define everything else (and measure everything else) relative to it.

Answer 5

its 3* 10^8 m/s=since its a fact, can't change the fact
i really don't know how the scientists came about with this but check the encyclopedia