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Discovery of Speed of Light and some methods to measure it

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From the ancient times to the modern days, It was a very long journey how concepts about light evolved with time and new experiments and observations. Today we all know Light is a electro-magnetic wave as well as bunch of photon particles.

The speed of light in vacuum is a fundamental constant. The accuracy of determination of speed of light really matters beacause it is the maximum possible velocity of a particle, according to the relativity theory. Many scientist Tried to calculate the speed of light from the ancient times. Their attempts and some successful method is discussed below.

Table of contents

  1. Ancient idea about speed of light
  2. Galileo’s attempt
  3. Beeckman’s attempt
  4. Romer's Determination
  5. Fizeau & Focult

1. Ancient idea about speed of light:

Empedocles suggested that the light from the Sun must take some time to reach the Earth. Aristotle's opinion was different. He suggested that light traveled instantaneously. Descartes also believed on that postulate that the speed of light is infinite or instantaneous.

2. Galileo’s attempt:

At first of all, Galileo Galilei attempted to measure the speed of light, in the early 1600s. Galileo and his assistant each stood on a different hilltop with a known distance between them and planned to open the shutter of a lantern and then for his assistant to open the shutter of a lantern as soon as he saw the light from Galileo's.Using the distance between the hilltops and his pulse as a timer, Galileo planned to measure the speed of light. He and his assistant tried this for different distances. But no matter how far apart they were, he could measure no difference in the amount of time the light took to travel.

his conclusion was that the speed of light was too fast.

3. Beeckman’s attempt:

In 1629, Dutch Scientist Isaac Beeckman gave an attempt to measure the speed of light. He placed mirrors at various distances from an explosions, created with the use of gunpowder. He also failed to measure the time difference between forming any two different image formation.

Till then Everybody’s experience supported Aristotle’s assumptions that speed of light is infinity or light travels instantaneously.

4. Romer's Determination by observing eclipses of Io (moon of Jupiter):

In 1676, Danish astronomer Ole Romer's method to measure the speed of light was demonstrated that light has a finite speed, for the first time. To understand his method we need some prerequisites.

Io is the innermost of the four moons of Jupiter. Time required for one Revolution around jupiter is 42½ hours. And the plane of its orbit and the plane of Jupiter's orbit around the sun both are nearly equivalent if we represent it in a 3D coordinate system. This means that IO passes through some shadowed region of its orbit so that eclipse can be observed.


Romer listed about sixty observations of eclipses of Io from 1668 to 1678.

Romer observed that the time elapsed between eclipses was not constant. there a slight variation with the different times of years when the data was taken. He was fully confident that the orbital period of Io was not actually changing. This was an observational effect. At that time the trajectory (not so accurate) of Earth and Jupiter was available to him. He noticed when Earth and Jupiter were moving away from each other always it takes longer interval of time between two successive eclipses. Conversely, when Earth and Jupiter were moving closer to each other it takes shorter interval of time. This reasoning was a satisfactorily explanation for finite speed of light.


From the known position of Earth and Jupiter in their orbits L(or K) can be found for each of the positions while the various eclipses is occuring. The distance between Earth and Jupiter can be calculated using standard trigonometry.


rj=k×a, distance between Earth and Jupiter
d=a^2(1+k^2-2kcos θ)

When we see eclipse of Io when Earth is at point L. We can see another eclipse of Io when earth is at point K. and get a extra time of Δt than the time interval should be, according to the calculation. So, Speed of light, C=d/Δt.

Romer found the most accurate value of k possible using those thirty observations of eclipses of Io from 1671–73 he had. The time he calculated for light to travel a distance a(the radius of Earth's orbit) was 11 minutes. But Romer never gave a value of speed of light. With his methods, Huygens estimated that
C=110,000,000 toises/second=214393994 m/s.

In the 17th century two legendary scientists attempted to explain Light and its phenomenons with their hypothesis.

One of them is Christian Huygens who explained light as a wave and explained reflection, refraction, double refraction with his theory of wavefront and wavelets. According to him, after refracting on a water surface speed of Light gets reduced. Speed of light is slow in water as a medium then air. So, Light bends towards normal.

The another scientist was Sir Issac Newton who thinks that light is flow of corpuscular particles and explained reflection and refraction. According to him, in the case of refraction, Light bends as water molecules apply larger force than air molecules. So, speed of light in water is higher than in air.

In that time there was no experiment available to determine speed of light in air or in water.

5. Fizeau & Focult

Now two french physicists Hippolyte Fizeau and Leon Foucault came one after one with their experimental set-ups.

Fizeau desiged a rotating toothed-wheel to make some cut in the continuous flow of light and placed this set up before a mirror to retrace the path of light. At an perticular angular velocity of that rotating wheel the light does not reached to the source after getting reflected. half of the lights are blocked at that time when light beam was crossing the wheel 1st time (source to the mirror). Left of the beam was blocked when It was returning to the source from mirror. From there he calculated the speed of light, C = 315000 km/s. But, His set-up was 8.6 km long.

In Foucault's design if we rotate an miror with a angular velocity ω location of the image of the source will be displaced. From that he calculated the speed of light,
C = 2.98×10^8 m/s. His experimental set-up was small enough to calculate the speed of light in any other medium. We can measure speed of light in any medium by putting the transparent material between two mirrors.

When speed of light was measured in air and in other denser liquid medium like water it was found that speed of light in water is lesser than in air or vacuum and it contradicted newton’s corpuscular theory.

Discovery of Speed of Light and some methods to measure it
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