How is the speed of light measured?
601530
Agree with terror_2287.\nThe speed of light is something basic. You will use speed of light to measure 'a meter', not to use 'meter' to measure speed of light.\nThink about it. Where did "one meter" come from? Before 1983, it was the standard meter placed somewhere in Europe (sorry, didn't remember where is it :D). But you know that even a very, very precisely cut rod of metal can still have error to the size of an atom, which is NOT negligible in quantum physics. So we must have a better definition and speed of light came up. A meter is the distance light traveled in 1/299792458 sec in vacuum. => speed of light (in vacuum) is 299792458m/s, with no error, no better measurement, no digit after the '.' And if you have your speed of light like this 299792458.12m/s or 299792458.00m/s you are having misconception.\nOf course we can, and have to, measure the speed of light in media, not vacuum. We do that by comparing the speed of light in the medium to that in vacuum (not by how many "standard meter" per second).\nAn easy and rather precise way to measure the speed of light in media involve the use of interferometers, with the earliest and still be in use is those of Reilegnh and Michelson which has good axplanation in Wiki (link below)\nRahul's way of measuring the speed of light with a microwave oven is rather like measuring the speed of sound, using stop wave. But there is a mistake: You will only get 'the meter', not the speed. You will have very exact 'centimeters' that way. Yea, that's a good way to 'recalibrate' your ruler, I think. But you will not get anything out of 'speed of light'.
495659
The accurate speed of light is 299,792,458 m/s.
666698
The speed of light in a vacuum is exactly 299,792,458 metres per second (or 1,079,252,848.8  kilometres per hour, which is approximately 186,282.397 miles per second, or 670,616,629.4 miles per hour). \nThe speed of sound c (from Latin celeritas, "velocity") varies depending on the medium through which the sound waves pass.
932886
the speed of light = 299 792 458 m / s or 1079252848800 M/H\ncircumference = 2*Pi*R \nso 2* 3.14159*R= 107925848800M/H\n1079252848800/(2*3.14159)=R\nRadius= 1.71768571 × 1011(<--- times 10 to the 11th power) meters
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
126628
The speed of light in a vacuum is exactly 299,792,458 metres per second (or 1,079,252,848.8 km/h, which is approximately 186,282.397 miles per second, or 670,616,629.4 miles per hour; sometimes roughly 300,000 km per second). \nmeasured with a laser at a known distance x time
474792
(m)\n\nThe speed of light in a vacuum is exactly 299,792,458 metres per second (or 1,079,252,848.8  km·h-1, which is approximately 186,282.397 miles per second, or 670,616,629.4 miles per hour). This value is denoted by the letter c, reputedly from the Latin celeritas, "speed", and also known as Einstein's constant. Note that this speed is a definition, not a measurement, since the fundamental SI unit of distance, the metre, has been defined since 1983 in terms of the speed of light—one metre is the distance light travels in a vacuum in 1/299,792,458 of a second. 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.
219832
The speed of light is indeed a constant.  What's important is that it is a constant to every observer.  If you are traveling toward a light source at 1000 m/s, you will measure the light's velocity at 300,000 m/s.  If you are traveling AWAY from the light source, it will still measure 300,000 m/s.\n\nLight IS the absolute speed limit. (Of this universe anyway)
727511
In metric units, c is exactly 299,792,458 metres per second (1,079,252,848.8 km/h). Note that this speed is a definition, not a measurement, since the fundamental SI unit of length, the metre, has been defined since October 21, 1983 in terms of the speed of light: one metre is the distance light travels in a vacuum in 1/299,792,458 of a second. Converted to imperial units, the speed of light is approximately 186,282.397 miles per second, or 670,616,629.384 miles per hour, or almost one foot per nanosecond.\n\nand everything about speech of light in this site\nhttp://en.wikipedia.org/wiki/Speed_of_light
263084
The speed of light in a vacuum is constant.\n\nThe speed of light varies in other media.  It varies from medium to medium, such as glass versus copper.  It also varies within a medium depending mainly upon the density of the medium.  For example, the speed of light through air varies with the density of the air.
104857
The speed of light varies depending on the medium it is passing through. The optical density of the material it is passing through is used to find the speed of light through the material. \n\nWhere c is a constant is in a vacuum. If that is what you are referring to, I have no idea.
13401
It's true that the speed of light is constant no matter what speed at which you or the source is moving.  This is a fundamental postulate of relativity, and there is no explination.  It's just accepted because our experiments indicate that it is true.  You used the formula "lights speed + X = actual speed"  this type of velocity addition is called "Galilean".  It turns out that speeds don't actually add that way.  Far out.\n\nWhere is the rest of the energy? Good question.  The frequency of the light you observe from the sun DOES depend on how fast you and it are moving.  The energy of each photon is given by E = hf, where f is the frequency of light.
54308
As you travel faster and faster, time for you slows down, and length contracts. From your point of view, when you measure the speed of light, it will always be the same. An observer standing still with respect to you and the beam of light could measure your speed at 185000 mps and the speed of light as 186000 mps, and say the difference in speed is 1000 mps. But that is relative to them. The speed of light will always be the same for whoever measures it, no matter how fast they are moving.\n\nSee the website below for more about relativity.
1187636
Light always travels at the same speed in a vacuum, the speed changes ever so slightly when it has to go through a different medium (water e.g.) but its so fast to begin with you wouldn't really notice it. 300,000 kilometers per second is beyond human comprehension, I mean you can only imagine how fast that is, you can't experience it or 'know' it.
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.
610852
Light exhibits the characteristics of a wave in some situations and the characteristics of a particle in other situations.  The wave theory asserts that light is a form of high frequency electromagnetic wave.  Basically, a changing magnetic field produces an electric field and a changing electric field produces a magnetic field resulting in a wave.  As far as the speed of light, the real question is how do you accurately measure something that is moving so fast?
208088
Actually light travels at the same speed no matter how fast it's source is traveling, nor which direction, as long there does not exist a certain level of gravity. Brightness of light however may be relative, except that you will have a definitive amount of photons in a given space...but we have no way of measuring that. The problem with the speed of light, however, is that gravity has an effect on it. Supreme amounts of gravity can actually attract photons and slow them down, or curve their direction towards the object. we do not know whether light actually orbits black holes because it's  constant inertia gives it enough centrifugal force, or if the light is actually sucked right into the object.\n\nYes everything is relative to something else. Your name spelled in English letters is even relative to other spellings of your name. Emotions are relative, hieght is relative, color is relative, even time is relative.\n\nThis is only true if you agree with Einstein's theory of relativity, but you couldn't say you didn't agree with it, because you would have to read all of it to understand. He has math that proves everything as relative, somehow.