Speed of Light in Mph, Kmh and in Different Medium

At what speed does the light travels? It has been a nagging question for mankind in last few centuries. Now it is a well-known fact that nothing can travel faster than light. And the exact speed of light in vacuum is 299 792 458 m/s or approximately 3,00,000 km/s, or 1,86,000 mi/s. If you want to compare it with the speed of your car, well, then in miles per hour, speed of light is about 670,616,629 mph. In kilometers per hour light’s speed is 1,080,000,000 kmh.

In vacuum, the speed of light is fixed — it never changes. Thus, speed of light in vacuum is used as a constant in science and it is denoted with the c letter. This constant plays central role in the following most famous equation from Albert Einstein:

E = mc2

Here E is energy, m is mass and c is the speed of light. This equation is called the mass-energy equivalence. It states that anything that has a mass also has equivalent amount of energy and vice versa.

But this is not really true that light is the fastest thing in the universe. The truth is that, in vacuum, all massless particles and field disturbances also travel at the speed of light. For example, electromagnetic radiation and gravitational waves also travel at the speed of light.

This is also true that any particle with some mass can only attempt to reach the speed of light but it will never really achieve it. So essentially, to achieve the speed equal to that of light, a particle has to be massless (i.e. zero mass particle).

Light-year is a popular unit of measuring astronomical distances. A light-year is the distance that light travels within a year.

Is Speed of Light Fixed?

We know that light travels at a speed of 299 792 458 m/s in vacuum — but would it also travel at the same speed in other environments? The answer is no! Just like sound waves and water ripples, the speed of light gets affected if there is hindrance in the way. As a result the speed of light will not be exactly 299 792 458 m/s in a earth’s atmosphere full of various gases and dust particles.

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speed of light : measurement and history

Nevertheless, earth’s atmosphere does not significantly slow down the light. However, while passing through a diamond, light loses more than half of its speed. Next time when you see a sparkling diamond, remember that the light is passing through it at a speed of about 1,24,000 km/s — much less than its real speed.

Speed of light in different medium

  • In vacuum speed of light is 3,00,000 km/s
  • Light passes through air at a speed of 2,99,000 km/s
  • Through water light travels at 2,25,000 km/s
  • Passing through transparent glass — the speed of light is 1,97,000 km/s
  • In diamond the speed of light is 124,000 km/s

History of the Measurement of the Speed of Light

A large number of investigators have attempted to determine the light’s speed throughout the history. As per the records, Galileo Galilei was the first person who tried to measure the light speed. He measured it to be around 333.5 km/s. Now we know that Galileo was way off the mark. But then his equipment consisted only of lanterns. The following table lists persons of science, their methods and accuracy to which they could determine the light’s speed. Take a note, how in 20th century the speed of light was so finely measured.
History of the Light Speed Measurement
Year Investigator Method Estimate
Kilometers
/Second
1667 Galileo Galilei Covered Lanterns 333.5
1676 Ole Roemer Jupiter’s Moons 220,000
1726 James Bradley Stellar Aberration 301,000
1834 Charles Wheatstone Rotating Mirror 402,336
1838 François Arago Rotating Mirror
1849 Armand Fizeau Rotating Wheel 315,000
1862 Leon Foucault Rotating Mirror 298,000
1868 James Clerk Maxwell Theoretical Calculations 284,000
1875 Marie-Alfred Cornu Rotating Mirror 299,990
1879 Albert Michelson Rotating Mirror 299,910
1888 Heinrich Rudolf Hertz Electromagnetic Radiation 300,000
1889 Edward Bennett Rosa Electrical Measurements 300,000
1890s Henry Rowland Spectroscopy 301,800
1907 Edward Bennett Rosa and Noah Dorsey Electrical Measurements 299,788
1923 Andre Mercier Electrical Measurements 299,795
1926 Albert Michelson Rotating Mirror (Interferometer) 299,798
1928 August Karolus and Otto Mittelstaedt Kerr Cell Shutter 299,778
1932 to 1935 Michelson and Pease Rotating Mirror (Interferometer) 299,774
1947 Louis Essen Cavity Resonator 299,792
1949 Carl I. Aslakson Shoran Radar 299,792.4
1951 Keith Davy Froome Radio Interferometer 299,792.75
1973 Kenneth M. Evenson Laser 299,792.457
1978 Peter Woods and Colleagues Laser 299,792.4588
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This was a brief explanation about how light travels and at what speed. If you know more interesting stuff related with this topic, please let us know in the comments section!