DIFFRACTION (PHYSICAL OPTICS)Chapter 36
SINGLE SLIT DIFFRACTION
WAVELENGTH DEPENDENCE
Longer wavelength, larger diffraction
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.
INTENSITY OF DIFFRACTION
CHARACTERISTIC OF DIFFRACTION
Central peak twice as wide
ANGULAR WIDTH
MINIMA
FIND THE DARK FRINGE
FIND THE SLIT WIDTH
DIFFRACTION THROUGH A CIRCULAR APERTURE
RESOLVABILITY FOR DIFFERENT α
Cannot be resolved Can be resolved
α
LARGE ANGULAR SEPARATION
α
Can resolve the two stars easily
SMALL ANGULAR SEPARATION
α
Cannot resolve the two stars
RAYLEIGH’S CRITERION
α
Just able to resolve the two stars when the maximum falls directly on the first minimum
RESOLVABILITY (RESOLVING POWER)
DIFFERENT APERTURE
EXAMPLE
θR
DIFFRACTION GRATING (MULTIPLE SLITS)
DIFFRACTION GRATINGS
Peaks much narrower than a double slit. N is the total number of slits.
DIFFERENT NUMBER OF SLITS
The width of the peaks decreases as N increases.
DIFFERENT COLORS (WAVELENGTH) DIFFRACT DIFFERENTLY
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).
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