| MadSci Network: Physics |
Hello shourya,
Thank you for your question. To answer it, we will first consider how Foucault's experiment can be used to determine the speed of light. Then, we will see why one would use only one of the two measured speeds.
The idea 1,2 behinds Foucault's experiment is really simple. Take a light source (nowadays, a simple laser works fine) a lens (to focus the light), a fixed mirror, and a rotating mirror. The light shines through the lens on the rotating mirror, hits the fixed mirror, bounces back, and hits the rotating mirror again. Because the mirror rotates, the light hits it at a different angle. This means the angle of reflection will also be different, and the light will shine into a direction left or right of the light source.
Assuming the fixed mirror is to the right of the rotating mirror, the beam will reflect to a position to the left of the light source if the mirror rotates clockwise and to the right if it rotates anti-clockwise. Provided you properly measure the distance from the light source properly (that is, on a circle with its centre at the axis of the rotating mirror), the distances left and right will be the same. Hence, measuring either is enough to measure the speed of light.
There is, however, an advantage to measuring the distance between the anti-clockwise and clockwise distance. It is likely that the largest measuring error is made in the measurement of the position of the reflected beam. By measuring the distance between the anti-clockwise and clockwise reflection, we have twice as much signal for the same noise, making the measurement more accurate.
To summarize the answer to your question: Yes, it is possible to carry out Foucault's experiment using only the distance measured with a clockwise or an anti-clockwise rotating mirror. However, the answer will be significantly less accurate.
Regards,
Bart Broks
Sources:
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