16.2 Roemer & Light Speed
Olaus Roemer (1664-1710) and Christiaan Huygens made one of the earliest attempts to measure the speed of light using astronomical data. The period of revolution for Jupiter's moon, Io, was well known. The moon would periodically disappear from sight, or eclipse, as it travelled behind Jupiter only to reappear on the other side every 42.5 hours.
However, depending on the time of year the eclipse was observed, the moon would appear 22 minutes behind schedule. Presumably, this time difference is caused by the extra distance the light has to travel when Jupiter and Earth are on opposite sides of the Sun. Given that the extra distance is equal to Earth's orbital diameter and the time it takes to travel that distance, Roemer was able to measure the speed of light to be 2.3 × 10 8 m/s. This value, although it was very close to the currently accepted value of 3.0 × 10 8 m/s, was rejected by the scientific community of the time, who assumed it to be much too high a figure because no such speed even remotely near had ever been measured.
Self-CheckAnswer the following self-check questions then click the "Check your work" bar to assess your responses. |
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The radius of Earth's orbit is 1.49 × 10 11 m. If the eclipse of Io took 22 minutes longer, demonstrate the calculations Roemer and Huygens used to calculate the speed of light.
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The value that Roemer and Huygens measured was considered much too large. In the 1700s, what was considered really fast?
Contact your teacher if your answers vary significantly from the answers provided here.
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Given
Required
the speed of light
Analysis and Solution
Paraphrase
Roemer and Huygens determined the value was 2.3 × 10 8 m/s.
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In the 1700s the fastest form of transportation was a horse, with a top speed of approximately 70 kph or 20 m/s. Even the fastest animal, a cheetah, had a maximum speed of only approximately 27 m/s. William Derham (1657-1735) was the first person to measure the speed of sound as 3.4 × 10 2 m/s. The speed of light measured at 3.0 × 10 8 m/s, which was about a million times faster than sound and was fast beyond imagination.
Even today, the space shuttle, the fastest human vehicle ever, has a maximum speed of 7800 m/s, which is approximately 0.0026% or twenty-six ten-thousandths of one percent of the speed of light.
ReadIn 1848, the first terrestrial attempt to measure the speed of light was done by Armand Fizeau using a toothed wheel. Read his experimental design on page 649 of your textbook. |
Self-CheckAnswer the following self-check questions then click the "Check your work" bar to assess your responses. |
SC 3.
What was the manipulated variable in Fizeau's experiment?
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What was the responding variable in Fizeau's experiment?
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What did Fizeau see during his first experiment?
Contact your teacher if your answers vary significantly from the answers provided here.
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The manipulated variable in Fizeau's experiment was the speed at which the toothed wheel turned. This is also called the wheel's frequency of rotation.
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The responding variable in Fizeau's experiment was whether or not the observer could see the light.
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In the first experiment, Fizeau would see the light reflected from the mirror. As the toothed wheel spun faster, the tooth would start to block the reflected light source so Fizeau would not be able to see the reflected light source. As the wheel continued to speed up, the light source would again become visible as the tooth would move all the way through the path of the light before the reflected light reached the toothed wheel.