chapter37

chapter37 - Chapter 37 Interference of Light Waves Wave...

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Chapter 37 Interference of Light Waves
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Wave Optics z Wave optics is a study concerned with phenomena that cannot be adequately explained by geometric (ray) optics z These phenomena include: z Interference z Diffraction z Polarization
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Interference z In constructive interference the amplitude of the resultant wave is greater than that of either individual wave z In destructive interference the amplitude of the resultant wave is less than that of either individual wave z All interference associated with light waves arises when the electromagnetic fields that constitute the individual waves combine
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Conditions for Interference z To observe interference in light waves, the following two conditions must be met: z 1) The sources must be coherent z They must maintain a constant phase with respect to each other z 2) The sources should be monochromatic z Monochromatic means they have a single wavelength
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Producing Coherent Sources z Light from a monochromatic source is used to illuminate a barrier z The barrier contains two narrow slits z The slits are small openings z The light emerging from the two slits is coherent since a single source produces the original light beam z This is a commonly used method
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Diffraction z From Huygens’s principle we know the waves spread out from the slits z This divergence of light from its initial line of travel is called diffraction
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Young’s Double-Slit Experiment: Schematic z Thomas Young first demonstrated interference in light waves from two sources in 1801 z The narrow slits S 1 and S 2 act as sources of waves z The waves emerging from the slits originate from the same wave front and therefore are always in phase
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Resulting Interference Pattern z The light from the two slits forms a visible pattern on a screen z The pattern consists of a series of bright and dark parallel bands called fringes z Constructive interference occurs where a bright fringe occurs z Destructive interference results in a dark fringe
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Interference Patterns z Constructive interference occurs at point P z The two waves travel the same distance z Therefore, they arrive in phase z As a result, constructive interference occurs at this point and a bright fringe is observed
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Interference Patterns, 2 z The lower wave has to travel farther than the upper wave to reach point P z The lower wave travels one wavelength farther z Therefore, the waves arrive in phase z A second bright fringe occurs at this position
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Interference Patterns, 3 z The upper wave travels one-half of a wavelength farther than the lower wave to reach point R z The trough of the upper wave overlaps the crest of the lower wave z This is destructive interference z A dark fringe occurs
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Young’s Double-Slit Experiment: Geometry z The path difference, δ , is found from the tan triangle z δ = r 2 r 1 = d sin θ z This assumes the paths are parallel z Not exactly true, but a very good approximation if L is much greater than d
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Interference Equations z For a bright fringe produced by constructive
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chapter37 - Chapter 37 Interference of Light Waves Wave...

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