How do we measure the speed of light?

Question

Doesn't it seem like something that can loop Earth 7.5 times in a second would be hard to record?

Answer 1

I measured it in my lab with some crappy equipment.  Essentially, we had a mirror spinning really fast, bounced a laser off of that, then down the hall, off a stationary mirror, back onto the spinning mirror, and into a microscope where the original laser is.  Since the mirror is spinning really fast, the reflected laser moves a little bit over.  We can measure the small distance, use some trigonometry, and figure out how fast the light must be moving to do that.  If the light was instantaneous, the reflected laser would not have moved at all.  My explanation may be a little bit hard to follow, so look up the Foucault experiment to read about it.

Answer 2

Through Maxwell's equations, it was shown that c = 1/sqrt(mu * epsilon) where mu and epsilon are the magnetic and electric permitivity constants respectively. We don't measure the speed of light directly since like you said it moves so fast. The speed of light is an absolute constant, a speed limit per se. By using coulumb's law and the magnetic force law, you can calculate the values of these constants through observations of the forces exerted on charges, and through that determine the speed of light using the earlier equation.

Edit: What you said though isn't changing c, it's changing c' where c' is the speed of light in another medium. It's not so different from the normal idea of index of refraction since the properties of the matter are slowing the light. Light can travel slower in glass, but that doesn't change the speed of light in a vacuum, c.

Answer 3

Oscilloscopes with rise times of nanoseconds. The light is reflected off of a mirror and the oscilloscope measures the time. Over 100 years ago, Heinrich Hertz made a rough calculation of 300,000 kps measuring the speed of electromagnetic waves, so imagine what we can do today.

Edit: it is defined by Maxwell's equations as above, but it can be measured fairly accurately in a lab.

Answer 4

Please visit the following site: http://www.colorado.edu/physics/2000/wav...

Also, RE:Bondman, C the constant for the speed of light is not as constant as was once thought:

http://www.hno.harvard.edu/gazette/1999/...

and also:

http://www.eurekalert.org/pub_releases/2...

Answer 5

There are a few ways to measure the speed of light, and when it was first done, it was a tremendous feat at the time.  One of the best ways it was done originally (a hundred years ago) was with two metal disks that looked like gears.  They had little notches in the them, just like gears.  They could spin on an axle and were set very far apart from each other.  Then, if the gears are aligned with each other so t he notches exactly do not line up, then light that made it through a notch in the first disk would be blocked by the second disk.  But if they were spun fast enough that the notch on the second disk had moved so that the light could pass through it by the time light made it from the first to the second disk, then light would pass through the two disks and you would get a signal (light) on the other end.  So by exactly knowing the speed of the two disks and the distance between them, you could measure the speed of light.  This still stands as one of the most impressive experiments ever done, since it would have been so difficult given the technology at the time.

However, at this point, you should know that the speed of light no longer needs to be measured.  A few decades ago, the meter was defined to by the amount of distance light travels in one second, so now light travels exactly 299,792,458 m per s and the length of meter is set so this is true.

Answer 6

You use the standard “formula” that distance equals rate multiplied by the time to traverse that distance: D=RT. Set up an experiment where light must traverse a known distance and measure how long that takes. Then solve for R=D/T.

The devil is in the details. Light moves pretty fast (about 300,000 km/s in vacuum) so either the distance must be very large or the time interval over which you measure the distance traversed must be very short. Roughly speaking, light takes about a nanosecond to move a foot. So, set up a “race course” of maybe the length of a football field (300 feet) and expect a round-trip time of about 600 nanoseconds for a pulse of light sent from one end to a mirror on the other end, reflecting back and arriving at your (very fast) photosensitive detector. If you can accurately measure the interval of time from when the pulse of light is emitted to when you observe the pulse reflection from the mirror, you will get a decent measure of the speed of light.

RADAR (RAdio Detection And Ranging) was developed during WWII based on this type of measurement, with the speed of light assumed to be a known quantity. Clearly it was no great feat to actually mark off a distance and measure the time for a radar pulse to go there and back. Well, maybe it was difficult sixty years ago, but it is fairly trivial today. Another neat trick is to sinusoidally modulate the amplitude of a continuous light source, such as a diode laser, and measure the phase difference in the modulation of the reflected beam. This yields a very accurate “time of flight” but, again, we assume the speed of light is already known and use the phase shift information to accurately calculate distance.

Answer 7

See the book,"college physics"by Dr.Harvey E.White.