how do u measure light speed?

Question

and how fast dose light travel.

Answer 1

If I had to design an experiment to measure the speed of light, I'd probably use a laser that sent pulses from a slot in the edge of a spinning disk and found the disk rotation speed that caused the laser reflection from a distant mirror to return through a different slot (in the same disk) to a detector when the disk had rotated through a known angle.

That is, I'd keep varying the speed of the disk rotation until my detector, usually occluded from the reflected laser pulse by the disk material, happened to observe the pulse through its slot in the disk.

If the distance between the disk & laser and the reflector is 10 km, it's a 20 km round trip for the laser pulse. If the detector's slot is 10 degrees antispinward of the detector when the pulse is sent out, then the detector would be aligned with its slot when the reflected pulse returns, if the rotation rate were 416.378 revolutions per second.

Professor (speaking to graduate student): How far away from here is the reflector?

Graduate Student (returning with the tape measure): 10 kilometers.

Professor: Good, well let's start spinning the wheel and sending out those laser pulses.

(Disk starts spinning.)

Professor: 100 RPS, 110 RPS... Say, boy, come over here and keep track of the rotation speed. I need to look for the returning pulse.

(Later...)

Professor: Ah ha! I saw it. What was the rotation speed just then?

Graduate Student: 416.4 revs per second.

Professor: Hm, I know that at pulse time the detector was 10 degrees spinward of its slot opening. So, in the time it took the disk to spin 1/36th of a revolution, the laser pulse traveled to the reflector and back again.

Graduate Student: Plus or minus some whole number of disk revolutions. Plus the possibility that the pulse came back through the emission slot instead of the detector's slot.

Professor: None of your sass. We checked for that before. The detector slot is deeper than the laser's slot, to prevent exactly that kind of confusion.

(Professor fiddles with his calculator.)

Professor: So at 416.4 RPS, it would take 66.71 microseconds for the 20 kilometer round trip, which means that the speed of light is 299808 kilometers per second.

The recommended value of the speed of light is 299792.458 km/sec, so the experimental error was 0.005 percent.

Answer 2

Hi. It was first 'measured' by watching the moons of Jupiter. When Jupiter was on our side of the Sun the moons would orbit, eclipse, pass each other, etc at a certain time. When Jupiter was farther away from us the moon timings would be delayed because light took time to travel the extra distance. So, your first answer is, sometimes with a telescope. Just under 300,000 kilometers per second is the speed.

Answer 3

light speed is faster than the speed of sound its way too fast to measure

Answer 4

186,000 miles per second.