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Thermodynamics

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Tunnel Experiment

In the years 1930-31, Michelson in collaboration with Pease and Pearson determined the velocity of light in vacuum. This was done by constructing a pipe one mile in length and three ft in diameter. The air was exhausted to a low pressure of 0.5 of mercury with the help of power exhaust pumps. By doing so, errors due to the refractive index of air were eliminated. Moreover, the brightness of the image increased considerably. A rotating prism B with 32 faces was used.

This experiment was completed by Pease and Pearson after Michelson’s death in 1933.

The 32-sided rotating mirror R, driven by an air turbine, reflected a beam from the slit C through a plane glass on the plane mirror M. The beam after reflection from the mirror M₂, was then reflected several times between the plane mirrors A and B. The focusing of mirror M₂ was arranged so as to form a conjugate image of C either on A or on B. The beam ratraces its own path as shown and finally the image is seen by the eye through the eyepiece E.

R was rotated as such a speed that one face just replaced the next, in the time required by the light to go and come back. When this is adjusted, the image will be found at the same place as that when R was stationary.

Let D be the distance traversed by light from one face of R and back to the adjacent face at the same position. Then,

c = D/t

Here, t = 1/32n, where n is the number of revolutions/second of R

The mean value of c found from three thousand observations was (2.99774 ± 0.00011) × 10⁸ m/s.