Lecture23

# Lecture23 - Lecture 23 More on Interference and Diffraction...

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Lecture 23: More on Interference, and Diffraction

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The Michelson Interferometer In the 1800’s, A.A. Michelson figured out a way to use interference of light to measure very small distances Idea is as follows: The point at the center of the screen will be either dark or light, depending on the difference between L 1 and L 2 Screen Incoming beam Mirrors Half-mirror “beam splitter” L 1 L 2 Beam 1 Beam 2
The distance traveled by beam 1 is 2 L 1 , while beam 2 travels 2 L 2 so the difference in distance is 2( L - L ) 2 L 1 - Λ 2 ( 29 = νλ; ν=0,±1,±2. .. 2 L 1 + δ-Λ 2 ( 29 = ν+ 1 2 λ 2 Λ 1 + Λ 2 ( 29 + 2δ = νλ + 1 2 δ= 4 λ for visible light is ∼ 5 x 10 -7 m

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The Interferometer and the Doppler Effect In the late 1800’s physicists assumed there must be a medium (the “aether”) to carry light waves Since the Earth is rotating on its axis, moving around the sun, etc., the aether must be moving with respect to the Earth Michelson and Morley realized they could use their interferometer to measure the speed of the aether in their lab
The idea is to build a rotatable interferometer: Screen Incoming beam L 1 L 2 Beam 1 Beam 2 Aether Wavelength of light in beams 1 and 2 will differ due to Doppler shift (moving medium) So interference will alternate between constructive/destructive as device rotates the number of times it alternates is related to the speed of the aether Screen Incoming beam L 1 2 Beam 1 Beam 2 Aether

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So Michelson and Morley do the experiment And find that the speed of the aether is… I imagine this was very disappointing to them and that they spent a while trying to figure out what was
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Lecture23 - Lecture 23 More on Interference and Diffraction...

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