Lab: determine the wavelength of a known light source using a Michelson Interfer

Question

Lab: determine the wavelength of a known light source using a Michelson Interferometer

(1) Suppose that the wavelength of the laser is 500nm and I turn the micrometer screw so that the scale says I have moved the mirror by 6.00 m. How many dark rings would I see move past a reference mark? Assume that the reference mark started in the middle of a bright ring.  Answer (1, 3, 9, 8, 11, 5, 10, 12, 6, 7).

(2) Suppose that there is a bright ring on a reference mark. You turn the micrometer knob and move the mirror by 3.56m. This causes 11.5 dark rings to pass by the reference mark ( you ended with a dark ring on the mark). What is the wavelength of the light? Type in your answer in nm but do not type the units.  

(3) Does a bright ring represent constructive or destructive interference?

Explanation / Answer

(1) When the mirror has moved by 6.00 um, the effective change in path of the laser between the two beams will be 2 x 6 um=12.00 um. This would result in a total of 12.00 um/500 nm = 24 fringe shifts and point to the bright fringe again. Then the number of dark fringes moved will be 24-1=23 dark fringes by moving the mirror by 6 um.

(2) When the mirror is moved by 3.56 um, the path difference between the two mirrors will be 2x3.56 = 7.12 um. This causes 11.5 dark rings to pass by the reference mark, then the wavelength will be 7.12 um/11.5 = 619.1 nm.

(3) The bright ring always represents the constructive interference. This occurs when the two interfering waves meet in phase so that the amplitude becomes maximum and provides the bright fringe.

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