Chapter 25

1. Chapter 25 Optical Instruments

2. Michelson Interferometer Section 7

3. 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

4. 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

5. 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

6. 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

7. 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”

8. 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

9. 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

10. 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