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# What is the speed of light?

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Introduction

### What is the speed of light?

The speed of light in vacuum is exactly 299,792,458 m/s (metres per second)

In 1983 the SI (Systeme International) defined a metre as:

• The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

When people refer to the speed of light, they refer to the definition above - the speed of light in a vacuum.

The speed of light is normally rounded to 300 000 kilometers per second or 186 000 miles per second.

The speed of light depends on the material that the light moves through - for example: light moves slower in water, glass and through the atmosphere than in a vacuum. The ratio whereby light is slowed down is called the refractive index of that medium.

Middle

Even in 1600 A.D. the famous Johann Kepler believed that the speed of last was instantaneous. According to him the vacuum of space did not slow the speed of light down.

Galileo was correct in saying that all this proved was that light moved faster than sound - not necessarily instantaneously.

#### Galileo - speed of light measured using lanterns

Galileo Galilei suggested in 1638 and carried out experiments in 1667 to try measure the speed of light. ( Pictures of Galileo Galilei )

Two people had to stand at least a mile apart. They both had covered lanterns. When one person uncovered his lantern, the other person had to uncover his lantern when he saw this. A third person measured the time between when the first and second lanterns where uncovered.

Conclusion

Even stranger was the fact that these predictions then became more accurate, till they were correct again. This strange cycle repeated itself again and again with great regularity.

Roemer realized that this time difference was caused by the difference between the distance between the earth and Jupiter. When Jupiter was closest to earth, the eclipses happened on time. The further Jupiter was away from the earth, the later the eclipses became - light had a longer distance to travel to earth and this took longer.

The size of the earth's orbit and Jupiter's orbit around the sun was known at that time.

Roemer used these figures to determine the distance between the earth and Jupiter for all his observations.

Roemer used these somewhat inaccurate distances (as it was known in 1675) to calculate the speed of light to be around 200 000 km/sec.

This student written piece of work is one of many that can be found in our GCSE The Earth and Beyond section.

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