historical measurements of the speed of light - explained simply and clearly

Hi everyone! This video is about the question: How fast is the light?
As early as 1600 Galileo Galilei tried to find out whether the speed of light is infinitely fast or whether it takes a certain time to get from one point to another. To do this, he carried out the following experiment. Two people are at the center of the experiment. They each hold a glowing lantern in their hand. These light sources can be opened and closed with one hand as required.
The person on the right now reveals the lantern. As soon as the person on the left sees the light, the person uncover their lamp as soon as possible. The person on the right can now measure the time between the point in time at which he uncovered the lamp and the point in time when the person on the left uncovered the lamp. To determine the reaction time, the experiment was repeated several times at a short distance.
Then the two people carried out the same experiment further and further apart and at night. The response time was always measured even at greater distances.
With this experiment, Galileo could not clearly say whether the speed of light is infinite or not.
Ole Römer took over the basic idea of the experiment from Galileo. In his day it was known that it would take the Jupiter moon Io 42.5 hours to orbit Jupiter. Whenever the moon Io is immersed in the shadow of Jupiter, it cannot be seen from Earth. So you had a light source that was very far away and kept going on and off. Since Jupiter and the earth take different times to orbit the sun, the distance between the earth and Jupiter and its moon Io varies. Romans now waited for the day on which the earth was very close to Jupiter and started the time when Io just emerged from the shadow of Jupiter. Now Römer waited until the moon Io completely orbited Jupiter, disappeared in the shadow and stopped the time when Io reappeared. It took the moon Io 42.5 hours to orbit Jupiter. Romans thought that he would have to wait 42.5 hours at a time to watch the moon Ios emerge from the shadow of Jupiter. He made a table with the times when he expected to leave. At that time, Römer pointed his telescope in the direction of Jupiter. Strangely, he found that the exit was becoming more and more delayed. After a few months, Io even stepped out of Jupiter's shadow several minutes later than calculated. How can that be explained?
When Romans started his measurement, Earth and Jupiter were relatively close together. After that, the distance between Earth and Jupiter grew larger and larger. The light therefore has to travel a longer distance and was therefore always seen a little later for the Romans. This made it clear that the speed of light cannot be infinitely great. From his observations, RÖMER determined a speed of light of around 220,000 km / s.
The first measurement of the speed of light on earth (terrestrial method) was made by Hippolyte FIZEAU in Paris in 1848.
The light from a light source is directed onto the edge of a gearwheel via a semi-transparent mirror. If it hits a gap there, the light can pass the gear, runs to the mirror several kilometers away and back, and hits the edge of the gear again.
If the gear does not rotate or only rotates very slowly, the light that runs from the source via the semitransparent mirror through a gap in the gear to the remote mirror can reach the observer's eye. The light is so fast that it can still pass the same gap in the gearwheel when it goes backwards. Each flash of light reaches the eye of the observer, who sees the lamp flash in time with the sequence of teeth when the gear wheel rotates very slowly. With fast rotation, our eyes can no longer keep up with the light-dark changes, it sees a "medium" brightness. If the speed of rotation of the gear wheel is increased, the case occurs that the light reflected by the distant mirror no longer hits a gap on the gear wheel, but hits the tooth which follows the gap through which the light ran on its way out. The observer now determines darkness. The light therefore needs the time to cover the path between the gearwheel and the mirror twice, in which the gap rotates so far that the reflected light hits a tooth. So you have a distance covered and a time. The value that Fizeau achieved using this method was approximately 315,000 km per second.
The rotating mirror method is a method developed by Léon Foucault in 1850/51 to measure the speed of light. A light source is arranged behind a projection screen with a passage opening so that its light falls on a rotating mirror.

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