How does scientists measure the speed of light?

According to Dr. Einstein, nothing can travel faster than light. Here we need to raise the other question, on what basis he told so? The special theory of relativity E=MC2 by him means mass equals to the energy of the body and vice versa in the presence the square of the light speed is taken as constant ‘c’. We will talk about Einstein’s theory in detail in the coming articles.

Light is the fastest thing in the universe. It can travel exactly 299 792 458 meters per second and took 8.3 minutes to reach earth from the sun. However, it is a difficult thing to find out the speed of light. So there are many astronomers and philosophers were worked on this from ‘Empedocles’ to Albert A. Michelson. At last ‘Michelson’ was the first person who received the Nobel prize in America for his work on finding the speed of light.

‘Empedocles’ was the first person in the 5th century who predicts that light has a finite speed. But ‘Aristotle’ doesn’t agree with this. He thought the light was instantaneous. ‘Euclid’ and ‘Ptolemy’ thought that light was coming from their eyes because of finding objects instantaneously whenever they open their eyes which are far away from them. After years, a philosopher ‘Alhazen’ stated that light is coming from objects to our eyes and also it has a finite speed.

Years later, the great philosophers ‘Kepler’ and ‘Descartes’ states that the speed of light is infinite. So many conflicts occurred whether the light is finite or infinite. The first experiment was occurred in 1629 by ‘Isaac Beeckman’ and he observed the flash of a cannon reflecting off a mirror about a mile. But he didn’t expect light can reflect more than that. So the experiment didn’t give an accurate result.

After that, ‘Galileo’ did an experiment in which two people with lanterns would stand a mile apart and turn on both lanterns at the same time. The time taken by light to reach the other person’s eyes were being recorded. But here the problem is they make a delay of 11 microseconds while turning on the lantern. So the result was not accurate and is inconclusive.

In 1676, Danish astronomer Olli Romer proved that the speed of light wasn’t infinite. He studied Io (the moon of Jupiter) and its in orbit around the planet, which took place every 1.76 days. There was a problem with his prediction on Io that is Io is not constant, which means Io is rotating around Jupiter. So this interrupts the experiment to record light speed. During the experiment, he noticed it was added less time when Jupiter comes closer and more time when it is farther away from Earth. ‘Huygens’ made this experiment in deep and recorded that light is traveling with a speed of 220 000 meters per second.

Next experiment conducted in 1727 by ‘James Bradley’ called Aberration of light. In the experiment, he observed that star from space is not constant and it rotates 360 degrees in a year. On this basis, if we were in January, the star was placed 0 degrees. And if we were in June, the star must complete its half revolution i.e., 180 degrees. But it is not what actually happens. Why because along with the star we are observing we are also in motion, which means Earth is revolving around the sun. This brings an error in the observation. James recorded the value of light speed is 303 300 KM’s per second.

Next scientist ‘Hippolyte Fizeau’ conducted his experiment time of flight in 1849. In this experiment, he used a light source, silver mirror, and a cog. Light from the source is passed from the cog and reached to the mirror and reflected the silver mirror via cog. So he noted the time taken by the light to go through the cog and came back from the cog. After doing some calculations, he concluded that the speed of light is nearly 313 000 KM’s per second. But it is not accurate, so there is a need to do more experiments on light speed.

To bring more accuracy ‘Foucault’ modifies this experiment by replacing the cog with rotating mirrors. The mirror can automatically be rotated during the experiment to get an accurate result. After some observations, he stated that light speed was 298 000 KM’s per second. Successfully, it gives somewhat better accuracy than Fizeau’s time-of-flight experiment.

Finally, Albert Abraham Michelson did an experiment in 1883 by using optical with laser technology he gave the nearly accurate result that is 299 796 and + or – 4 KM is not sure. He was awarded the Nobel prize in 1970 for his seamless efforts in finding the speed of light. Later, by upgrading technology, we can find the exact speed of the light is 299 792 458.2 meters per second.


  • The speed of light is not constant at every time, and it can travel at different velocities in different mediums.
  • When we achieve to travel with the speed of light, we are able to round the Earth 7 and 1/2 times in just one second.
  • It took 1.3 seconds for light to reach from Earth to Moon. And 8.3 minutes from Sun to Earth.
  • Stars are far away from us. So when we look at the sky, the star’s actions are not happening at an instant.
  • It took four and a half years for light to reach the nearest star except the sun.

‘Science’ is infinite, and so discovering things about speed of light is infinite. We can look forward to great discoveries.

Author: Aswarth

Hey Guys, I'm an Electrical Engineer who is interested to learn Technical stuff in deep and want to give you the best quality information on this blog.

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