Rotating Mirror Null Method

A. A. Michelson, an American physicist, spent many years of his life in measuring the velocity of light. The method devised by him in the year 1926 at the mount Wilson observatory is considered accurate.

Light from an arc after passing through a narrow slit S is reflected from one face of the octagonal mirror R. Then it is reflected from the small fixed mirrors B and C to a large concave mirror M₁. The point on the mirror C from where light is reflected is the focus of the concave mirror M₁. The light then travels as a parallel beam to another concave mirror M₂, and it is reflected to a plane mirror D at the focus of the concave mirror M₂. (The focal lengths of the concave mirrors M₁ and M₂ were nearly equal to 30 ft and was about 22 miles). The light is then reflected back to the concave mirror M₂, travels back to M₁ and is therefore reflected to a plane mirror F and is then incident on the face A’ of the octagonal mirror opposite to A. The final image is viewed through a micrometer eyepiece T with the help of a total reflecting Prism P. When the rotating octagonal mirror is stationary, the image is seen by light reflected from the top surface A’. When it is rotated the image disappears. The speed of rotatio n of R is suitably adjusted so that the image reappears and is seen in the same position as when R is stationary. The light reflected from A now arrives at A’ in the time taken by R to rotate through 45˚ or 1/8 of a revolution so that the next face is present at A. For this critical speed, the beam is reflected from the next face exactly along the same path as when the mirror was stationary.

If the distance travelled by light in its journey from A to A’ is equal to d, then

c = d/t

If R makes a revolution per second, then

t = 1/8n

c = d/(1/8n) = 8nd

Thus c can be calculated.

Michelson set up M₁ at an observing station on Mt. Wilson and M₂ at Mt. St. Antonio in California, at a distance of 22 miles. The velocity of the revolving mirror R was measured by stroboscopic comparison with an electrically maintained tuning fork. He obtained the value of c = 2.99797 × 10⁸ m/s.

Services: - Rotating Mirror Null Method Homework | Rotating Mirror Null Method Homework Help | Rotating Mirror Null Method Homework Help Services | Live Rotating Mirror Null Method Homework Help | Rotating Mirror Null Method Homework Tutors | Online Rotating Mirror Null Method Homework Help | Rotating Mirror Null Method Tutors | Online Rotating Mirror Null Method Tutors | Rotating Mirror Null Method Homework Services | Rotating Mirror Null Method