Interesting Facts I Bet You Never Knew About Speed of Light

What is the fastest object on Earth? Obviously, it the speed of light. But what is its speed? Is it really 300,000 km/s. We will check it out here, just read the article till the end  

Obviously, we see anything in this cosmos due to the interference of Light. Not only the interference, but there are also many more properties of light due to which we are able to see anything.  

But how fast it is? What is its responsiveness? And what time it takes to reach the information to you from your desktop, laptop, mobile or tablet?

So, today we all are going to know the fact related to these questions and their explanations related to the speed of light.

What is the Speed of Light? Is it really 300,000 kilometers in a second?

What is the speed of light? We have studied since our childhood, the speed of light is 300,000 km/s. Is it the actual speed of light? No, it is not the actual speed of light, it is just an approximation of the actual speed of light.

Now, here is a question, what is the actual speed of light? Hmm… any answer??? So,, the light travels at the rate of 299,792,458 meters in a second. It is really interesting. Isn’t it?

Now, convert this meter per second into kilometer per second. I know, you must know the rule of conversion from meter per second to kilometer per second. Yes! It is easy, just divide the meters by 1000 and you’ll get the kilometer. Now, doing this calculation on your own you’ll get 299792.458 km/s.

Now, it’s time for the interesting part. We are rounding off the speed of light in a vacuum by 207.542 km/s which is far greater than that of many other objects on this planet like sound which travels only at a speed of 332 m/s.

The fastest train on this planet, Shanghai Maglev runs at the fastest speed of around 0.119 km/s. Fastest aircraft, North American X-15 runs at a speed of 2 km/s. All these fastest objects are very much less than the speed of light in the vacuum which we neglect.

So, the conclusion is that we usually neglect the lesser parts for our convenience. Was it required? Yes, I think it was needed for us to remember the speed of light in vacuum.

Is the speed of light constant? Or it changes according to its medium of pass through?

We have just discussed, the velocity of light is 299792.458 km/s. But this speed is in a vacuum. What happens to the speed of life when it passes through a medium other than a vacuum?

Well, it usually got decreased when interfaces with a new transparent medium. Like, speed of light in air and the speed of light in glass is lower than the speed of light in vacuum, and we can calculate these speeds if we have the knowledge of refractive index for that medium.

For example, the refractive index of the water medium is 1.3. Now, we can easily calculate the speed of light in water medium using very basic formulae of refractive index which

n = c/v ……………………. Eqn 1

v=c/n  ……………………Eqn2


n= the refractive index of the medium

v= the speed of light in that medium (here water)

c= the speed of light in vacuum

So, from the above equation, we can calculate the speed of light in the water medium by simply dividing the speed of light in a vacuum by 1.3 which is the refractive index of water medium. By this calculation, you will get the speed of light in water as 230609.5831 km/s.

By using the same formulae, we can calculate the velocity of light in the air medium and also in the glass medium. The speed of light in a glass medium can be calculated by dividing the same by 1.5 ( the refractive index of glass medium)

Is light the fastest particle in the cosmos?

This is a very fascinating question, is there anything faster than the light? No, not at all, not in physical form. But, if we talk about a hypothetical particle, yes, there is one. In 1967 in a paper, Robert Ehrlich & Arnold Somerfield proposed a possibility of such particles and they named it Tachyon or Tachyonic Particle.

So, Tachyon is, hypothetically the fastest particle in the cosmos. But many physicists believe that no particle can exist on a speed faster than that of light because these kinds of particles do not follow many of the physical laws known to us.

If there is any possibility of the existence of these kinds of particles, they would be very useful in building a tachyonic antitelephone which may send the signal faster than the velocity of light, and this leads to a violation of causality. Till now, not a single experimental evidence in the support of such particles has been found. 

What is the symbol of notation for the speed of light? Is it “c” or something else?

We all know and denote the velocity of light in a vacuum with a symbol, lowercase “c”. Was it the same from the beginning? No, obviously not. So, what was the first used notation for the velocity of light in the vacuum? It has an interesting past.

Well, c was the first notation used for the velocity of light but it was later discovered to be used for some other constant which equals to the square root of two times that of the velocity of light in vacuum. “c” was first used in 1856 by Wilhelm Eduard Weber and Rudolf Kohlrausch.

In 1865, James Clerk Maxwell used the symbol “V” as an alternative for the speed of light. Then, after 29 years, Paul Drude, used “c” again, but now with its modern meaning.

Einstein also used the symbol given by Maxwell in his Original German-Language Papers on special relativity in 1905. But, in 1907, he switched to “c”, which then standardized as the symbol of the speed of light in vacuum.

The Speed of Light plays an important role in the definition of the unit of length.

We all know the moon is around 4 lac km away from earth and the sun is around 150 million km. But, how come we know to get this unit of distance measurement?

Definition of Metre

For measuring a distance is like measuring the length of a bar. So, before measuring any distance we must need to know the unit of length.

In the International System of Units, the unit of length is meter and the velocity of light plays a very crucial role in the definition of the metre. As it is defined below

“The meter is the length of the path traveled by light in vacuum during a time interval of 1/299792458 of a second.”

Definition of Light-year and Parsec

Now, this was for a short distance or we can say for a distance up to one astronomical unit, which is the distance between the sun to the earth. But, what about if we are measuring a distance more than the astronomical unit.

For longer distances, like stellar distances we usually implement the concept of light year and parsec. Now, what is this light year & parsec?

So, a light-year is a distance traveled by light in a year which can be easily calculated by multiplying the velocity of light in km/s and the seconds in a year. By calculating this we get the actual value of Light year which is

300,000*365*24*60*60= 9.46 x 1012kilometres.

From this definition, the velocity of light directly affects the light-year. Now, Parsec, a parsec is a unit of length for objects far-far away from us. It equals to 3.26 times that of a light-year.

So, from all the above definitions, it is cleared that it is a very important factor in measuring length, whether it is few meters or thousands of light-years.


From the above post we have studied just now, we can conclude that the speed of light is a very important factor in our day to day life and it very fascinating to study it.

Expecting your support and positive remarks to create more posts like this.

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