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DIFFRACTION (PHYSICAL OPTICS)Chapter 36
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SINGLE SLIT DIFFRACTION
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WAVELENGTH DEPENDENCE
Longer wavelength, larger diffraction
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NEAR-FIELD AND FAR-FIELDFresnel diffraction (near-field):Source, obstacle, screen are all close to each other.Fraunhofer diffraction (far-field):Source, obstacle, screen are far from each other. Light rays can be considered parallel to a good approximation. This is the case we will study.
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INTENSITY OF DIFFRACTION
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CHARACTERISTIC OF DIFFRACTION
Central peak twice as wide
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ANGULAR WIDTH
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MINIMA
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FIND THE DARK FRINGE
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FIND THE SLIT WIDTH
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DIFFRACTION THROUGH A CIRCULAR APERTURE
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RESOLVABILITY FOR DIFFERENT α
Cannot be resolved Can be resolved
α
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LARGE ANGULAR SEPARATION
α
Can resolve the two stars easily
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SMALL ANGULAR SEPARATION
α
Cannot resolve the two stars
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RAYLEIGH’S CRITERION
α
Just able to resolve the two stars when the maximum falls directly on the first minimum
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RESOLVABILITY (RESOLVING POWER)
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DIFFERENT APERTURE
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EXAMPLE
θR
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DIFFRACTION GRATING (MULTIPLE SLITS)
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DIFFRACTION GRATINGS
Peaks much narrower than a double slit. N is the total number of slits.
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DIFFERENT NUMBER OF SLITS
The width of the peaks decreases as N increases.
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DIFFERENT COLORS (WAVELENGTH) DIFFRACT DIFFERENTLY
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EXAMPLE: WHITE LIGHT ON A GRATINGFind the angular spread of the first order bright fringe when white light falls on a diffraction grating with 600 slits per millimeter. The wavelengths of the visible spectrum are approximately 400nm (violet) to 700nm (red).