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Chapter 25. Optical Instruments. Michelson Interferometer Section 7. Michelson Interferometer. The Michelson Interferometer is an optical instrument that has great scientific importance It splits a beam of light into two parts and then recombines them to form an interference pattern

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Chapter 25

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Chapter 25 l.jpg

Chapter 25

Optical Instruments


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Michelson Interferometer

Section 7


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Michelson Interferometer

  • The Michelson Interferometer is an optical instrument that has great scientific importance

  • It splits a beam of light into two parts and then recombines them to form an interference pattern

    • It is used to make accurate length measurements


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Michelson Interferometer, schematic

  • A beam of light provided by a monochromatic source is split into two rays by a partially silvered mirror M

  • One ray is reflected to M1 and the other transmitted to M2

  • After reflecting, the rays combine to form an interference pattern

  • The glass plate ensures both rays travel the same distance through glass

Active Figure: The Michelson Interferometer


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Measurements with a Michelson Interferometer

  • The interference pattern for the two rays is determined by the difference in their path lengths

  • When M1 is moved a distance of λ/4, successive light and dark fringes are formed

    • This change in a fringe from light to dark is called fringe shift

  • The wavelength can be measured by counting the number of fringe shifts for a measured displacement of M

  • If the wavelength is accurately known, the mirror displacement can be determined to within a fraction of the wavelength


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Luminiferous Ether

Classical physicists (Maxwell, Hertz, etc.) compared electromagnetic waves to mechanical waves

Mechanical waves need a medium to support the disturbance (air, water, string, etc.)

The luminiferous ether was proposed as the medium required (and present) for light waves to propagate

Present everywhere, even in empty space

Massless, but rigid medium

Could have no effect on the motion of planets or other objects


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Verifying the Luminiferous Ether

Associated with the ether was an absolute frame of reference in whichlight travels with speed c

The Earth moves through the ether, so there should be an “ether wind” blowing

If v is the speed of the “ether wind” relative to the Earth, the observed speed of light should have a maximum (a), minimum (b), or in-between (c) value depending on its orientation to the “wind”


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Michelson-Morley Experiment

First performed in 1881 by Michelson

Repeated under various conditions by Michelson and Morley

Designed to detect small changes in the speed of light

By determining the velocity of the Earth relative to the ether


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Michelson-Morley Equipment

An interference pattern was observed

The interferometer was rotated through 90°

  • Used the Michelson Interferometer

  • Arm 2 is initially aligned along the direction of the earth’s motion through space

  • Should observe small, but measurable, shifts in the fringe pattern as orientation with the “ether wind” changes

Active Figure: The Michelson-Morley Experiment


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Michelson-Morley Results

Measurements failed to show any change in the fringe pattern

No fringe shift of the magnitude required was ever observed

The addition laws for velocities were incorrect

The speed of light is a constant in all inertial frames of reference

Light is now understood to be an electromagnetic wave, which requires no medium for its propagation

The idea of an ether was discarded


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