How is the speed of light calculated?
101086
One example to measure the speed of red light is to use equipment that includes a LED (light-emitting diode) that emits a regular series of pulses of red light that are only 20 nanoseconds in duration.\n\n    (A nanosecond is one-billionth of a second or 1/1,000,000,000 second. It is also written as 10-9 seconds, 10^-9 or 1e-9, where the -9 indicates the number of zeros in the denominator.)\n\nThat means the pulse of light blinks on for 20 billionths of a second or 20/1,000,000,000 second and blinks 40,000 times per second. Having such a short pulse allows the scientist to measure the difference in time it takes to travel two different paths. If the duration was longer, the distance traveled would have to be longer.\n\nBy splitting the light beam with a half-silvered mirror, one beam travels to a mirror 10 meters away and then back to a photodiode detector. The other beam is reflected off a mirror only a few centimeters away. The time difference for the two beams is about 67 nanoseconds, which can be displayed on a regular dual beam oscilloscope.\n\nThe total distance the light travels is 20 meters, which equals 0.02 kilometer (20/1000).\n\nThe speed of light is then:\n\nc = 0.02 kilometers / 67*10-9 seconds = 298,500 kilometers per second\n\nThis is a fairly accurate reading and pretty close to the actual speed of 299,792 km/s.
1166593
The speed of light does depend on the medium in which it travels. See refraction \nRefractive Index = Speed of light in Vacuum / Speed of light in Medium\n\nRefractive index = c / v    where v<c  where c= 299,792.458 kilometer per second or 186,282.397 miles per second. http://en.wikipedia.org/wiki/Speed_of_light\n\nSee http://www.onlinephysicshelp.com/refraction.htm \n\nCURIOUS  is that the speed of light depends only on the medium it travels through. So when it travels in vacuum it is c, when in glass it is v and when it returns to vacuum it is again c.\n\nWhile The Michelson-Morley Experiment), by measuring the speed of a beam of light in the detection and against direction of the Earth's rotation, has demonstrated in 1887 no change in speed of light. That suggested that light wave propagates without a medium. \nhttp://en.wikipedia.org/wiki/Michelson-Morley_experiment  \n\nHowever \nmedium does have effect on its propagation: " The speed of light through a transparent medium (that is, not in vacuum) is less than c; the ratio of c to this speed is called the refractive index of the medium." http://en.wikipedia.org/wiki/Speed_of_light \n\nI hope that helps.
1186727
It is a failing of the current theories of physics that the values of the key fundamental constants - of which c is one - cannot be derived but have to be measured and inserted into the equations. In other words, no-one knows what determines the speed of light.\n\nIt is true that the permittivity and permeability of free space are related to the speed of light, but the relationship is the other way around - they are what they are because the speed of light is what it is.\n\nThere have been two sources of thoughts that the speed of light may vary over time. One is theoretical, because it may help the sums work better when describing the early universe (i.e. moment after the big bang).\n\nThe other is experimental, and is controversial as it has not been independently confirmed (see link). This involves looking at light from galaxies so far away the light took billions of years to reach us - ie we are looking back to the early universe. But the measurements show a change in the fine structure constant, and not c directly, so there could be other sources of the discrepancies if the measurements are correct.
11469
the speed of light = 299 792 458 m / s
11463
The changes propagate at the speed of light, so I guess the time taken is,\n\nt = r/c where c = speed of light
229632
The speed of light pertains to E&M waves... You can have a wave on a rope for example... it will not be travelling at the speed of light.  To generalize, just state it as 'velocity'\n\nwavelength = velocity/frequency\n\nfor units in this case, x millimeters = (y millimeters/second)/(z cycles/second)
40471
E=mc^2 \nEnergy = mass * (speed of light)^2, c = the speed of light (approx. 186,000 miles per second).
974250
This page gives a very nice table listing who, when, how, how accurately the speed of light has been measured including the first time when Olaus Roemer did it in a really neat way 330 years ago!\n\nhttp://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/measure_c.html
182197
Disregarding the pseudoscientific ramblings above, use Einstein's energy equation which states that the energy of a particle is the magnitude of its momentum 4-vector, E^2 = (mc^2)^2 - (pc)^2. Since the photon is massless, this becomes E^2 = (pc)^2, and we get the momentum of a photon as directly proportional to its energy p = E/c. \nDon't be reluctant to use the equations, and don't try to shoehorn Newtonian mechanics where it does not belong (speeds at or near the speed of light). In this case, you are using the fallacy that classical momentum mv applies to a relativistic object. For more information about similar fallacies, see the second source below.
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E=m c^2\nEnergy=Mass &#92;times (speed of light)^2
344500
Yes the difference between the two are still the speed of light -- it's because at speed of light, or even just factions of the speed of light "normal" Neut on physics stops and only relativity theory explains what happens.  At speed of light time no longer move at the same rate, so the two object will not have same reference in time, and since speed is distance over time, the comparison make little sense to the normal perception of how things work.\n\nAn that ultimately explain why the difference between the two is not 1000 miles per hour.
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As light passes through more and more optically dense media, the speed at which light wave front propigates slows down and is bent.\n\nThe amount which the light ray is bent depends on the indicies of refraction of the two media (the one the light is leaving and the one it is entering).  Using Snell's law, we can calculate the angle as,\nn_1 * sin(angle 1) = n_2 * sin (angle 2)\nwhere n is the index of refraction and angle is the angle between the light ray and the normal line on the medium's surface.\n\nKnowing the index of refraction, we can calculate the speed of light in a particular medium.\nv = c / n\nwhere v is the speed of light through the medium, c is the speed of light in a vacuum, and n is the index of refraction.\n\nAs light passes from one medium to another, the speed at which it propigates slows down and the wavelength changes, however, the frequency of the light remains constant.
71455
http://en.wikipedia.org/wiki/Speed_of_light#Measurement_of_the_speed_of_light
495659
The accurate speed of light is 299,792,458 m/s.
575949
Einstein postulated that the speed of light would remain the same regardless of the inertial frame in which it is measured.  This contradicts classical physics, since if an object is moving at a velocity v, as measured in an inertial frame, and that inertial frame is moving with a velocity v2 with respect to a second inertial frame, then you should take the vector sum of the two velocities to determine the velocity of that object with respect to the second inertial frame.\n\nEinstein postulated that within an inertial frame the laws of classical physics held.  So, to make things work out Einstein calculated that time and space are distorted with velocity.  It is as though you and your brother are of different heights, but you both want to be the same height, so you take a rubber ruler and stretch it so you get the same measurement for the two of you.\n\nIn this way you always measure the speed of light as the same value, no matter how fast the source of the light is moving with respect to you.  You adjust the rubber ruler so that what you want to believe remains true.
1169043
The speed of light in a vacuum is exactly 299,792,458 metres per second or 1,079,252,848.8  kilometres per hour. Converted to approximate imperial units, it is 186,282.397 miles per second, or 670,616,629.384 miles per hour.
996962
The speed of light is a constant (186,000 mi per second) and does not change based on what it is travelling through.