how do scientist measure light speed per second?

Answer 1

Light speed is light speed; they only had to measure it once.

go read this:

http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html

Answer 2

Several ways
One is to observe moons going around planets and time them - the timing is off when they are closer than further and the difference is due to the speed of light. This was one of the early ways.
Another is to send a beam of light a considerable distance (like to a mountain), mirror it back and when it gets back have it hit mirrors on a spinning drum. Because of the time it takes, the mirrors have moved and if the light is flickered or by measuring interferance, the speed can be determined from the time and the distance. Also an older method.
If the frequency of light can be accurately measured (recently because of the need for very accurate clocks) then the distance between wavelengths can be measured in interferance wave lengths and the speed of light is the product.
Electronics have now gotten fast enough that a laser beam of light can be turned on and off in a short time and the length of the beam measured (a nanosecond has been called a light foot because light moves a foot in a nanosecond)

Answer 3

Try this experiment at home...

Butter some bread and put it in the microwave oven having removed the turntable. You want the bread stationary in the oven so place it on e.g. an upside down plate over the rollers.

Heat it for about 10-15 seconds and you will see lines of melted butter on the bread. Measure the distance apart the lines are and double this distance. This will give you the wavelength of your microwave's radiation.

If you look on the back of your microwave you will see the frequency of your microwave. As v=f x lambda (wavelength), multiplying the wave length by the frequency will give you an approximation of the speed of light

Answer 4

That is a good question, I will have to research it.

Answer 5

With a very fast stopwatch