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# Electrical Engineering/Speed of Light Computation.

Question
Dear Prof Cleggsan

en.wikipedia.org/wiki/Speed_of_light
www.speed-light.info
galileoandeinstein.physics.virginia.edu/lectures/spedlite.html

The Speed of Light = 299792458 m/s - Universal Physical constant.

1 What is the method used for computing the speed of Light and how this value for the speed of light given above is computed and derived ?.

2 Is there a single method or there can be several methods to calculate
the Speed of Light and derive the above value?.

Thanks & Regards,
Prashant S Akerkar

Speed of light has been measured many different ways over the years; as time went by those measurements kept getting closer and closer:

http://www.speed-light.info/measure/speed_of_light_history.htm

Probably Michelson was the first to get it quite accurate with his new approach:

Here is a modern, scientific way that can be done in a physics laboratory:

Modern methods

With modern electronics, the speed of light can now be measured in a physics lab.

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.

(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.)

That 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.

By 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.

The total distance the light travels is 20 meters, which equals 0.02 kilometer (20/1000).

The speed of light is then:

c = 0.02 kilometers/67*10-9 seconds = 298,500 kilometers per second

This is a fairly accurate reading and pretty close to the actual speed of 299,792 km/s.

(The above method is taken from http://www.school-for-champions.com/science/light_speed.htm#.VK1wBWOt1Cw )

Electrical Engineering

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#### cleggsan

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