1. Measuring the Speed of Light

2. Galileo
Galileo’s experiment to measure the speed of light. One assistant opens a shuttered lantern and Galileo starts timing. A second assistant stands a distance away and opens the shutter on his lantern when he sees the light of the first lantern. Galileo stops timing when he sees the light of the second lantern.

3. Light if not instantaneous is extraordinarily rapid.
Galileo’s Conclusion
Light if not instantaneous is extraordinarily rapid.

4. Roemer
Roemer noticed that the motion of the moons of Jupiter seemed to change depending on the time of year. He realized that it wasn’t the motion that was changing it was the time it took the light to reach Earth that was changing. Notice that as the Earth and Jupiter move around the Sun the distance between them changes. Roemer calculated the distances and the time and then used this information to calculate the speed of light.

5. Fizeau
Fizeau measured the speed of light by using a toothed wheel. The toothed wheel was spun and at a specific speed the image in the viewing piece would disappear. This meant that the incident beam from the half-silvered mirror passed through a gap in the wheel but the reflection from a distant mirror was blocked by one of the teeth. By knowing the speed of the wheel Fizeau calculated the time it took light to travel from the wheel to the mirror and used that to calculate the speed of light.

6. Foucault
Similar to Fizeau, but used a spinning mirror instead of a toothed wheel.
Foucault also measured the speed of light in water by placing the spinning mirror at one end of a tank of water and the stationary mirror at the other end.

7. Michelson
Michelson (who was the first American to win the Nobel Prize in Physics) used Foucault’s idea of a spinning mirror but extended the distance by placing the mirrors on mountains 35 km apart. This minimized the percent error produced by lengthening the time the light travels.

8. Speed of Light
3 x 108 m/s
300,000 km/s
186,000 miles/s