Electromagnetic Spectrum

The Speed of Light

The speed of light is one of the most important constants in nature. Our understanding of light has gone through interesting twists and turns over the centuries. Aristotle believed that we could see in the daytime because the air was transparent, and that we could not see at night because the air then was opaque. Other Greek philosophers Euclid and Ptolemy postulated that light emanated from the eyes (like death rays), and so they argued that light must have an infinite speed because we can see the far away stars immediately when we open our eyes.


Numerous experiments were devised to measure the speed of light, but for hundreds of years, all failed. For example, around 1630, Galileo Galilei conducted an experiment where two people stood on mountain peaks a mile apart, covering and uncovering a lantern and observing the time delay. No time delay was detected. And so, it was believed that light had an infinite speed or its speed was far too fast to measure.


The first successful determination of the speed of light came in 1676. The Danish astronomer Ole Römer, while working at the Royal Observatory in Paris, proposed that he could explain the slight “errors” observed in the orbital period of Jupiter’s moon, Io. Jupiter and Io, one of its moons. NASA image.Because Earth’s orbit around the Sun periodically brings Earth closer and further from Jupiter, the distance light has to travel, and thus the time it takes, varies. Römer correctly proposed that the changing distance between Jupiter and the Earth, and thus the changing time it took light to traverse the distance, caused the so-called errors. His logic was correct and he determined that the speed of light was very fast, but not infinite. On learning of Römer’s successful measurement, Isaac Newton deduced that not just white light but all colors of light traveled at the same speed. And he was correct.


In 1850, the French physicist Leon Foucault, determined the speed of light with surprising accuracy by measuring the angle of reflected light from a mirror that rotated with a known velocity. This experiment resulted in an estimate for the speed of light with only a 5% error from today’s known value of 299,792,458 m/s in a vacuum. For many calculations a value of 300,000,000 m/s = 3 x 108 m/s is adequate.


Finally, why is the letter c used as the symbol for the speed of light? It comes from the Latin celeritas which means swiftness.

Speed of light

= c = 3 x 108 m/s

How long does it take light to travel from the Sun to the Earth, and the Moon to Earth? Calculate it and then select the correct answer:


Sun to Earth:

1.3 seconds
8.3 minutes
2.8 years

Moon to Earth:

1.3 seconds
8.3 minutes
2.8 years

Sun to Earth = 8.3 minutes
Moon to Earth = 1.3 seconds

Remember from Equation 1.1 that v = d/t so that t = d/v

From Sun to Earth: t = 1.5 x 1011 m / 3 x 108 km/s = 0.5 x 103 s = 500 s = 500/60 s/min = 8.3 minutes

From Moon to Earth: t = 3.8 x 108 m/ 3 x 108 m/s = 1.3 s

How did you know the Earth-Sun and Earth-Moon distances? You googled them, same as me!

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