MK Fisika Dasar 2_Bab16
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Transcript of MK Fisika Dasar 2_Bab16
MK FISIKA DASAR 2ENGE6000044 SKS
Rachmat AndikaMultiferroic Research GroupDepartemen FisikaFMIPA - UI
DIFFRACTION PATTERNS
OUTLINE Introduction to Diffraction Patterns Diffraction Patterns from Narrow Slits Resolution of Single-Slit and Circular Apertures The Diffraction Grating Diffraction of X-Rays by Crystals
The diffraction pattern that appears on a screen when light passes through a narrow vertical slit Intense central band : central
maximum Less intense additional bands : side
maxima or secondary maxima A series of intervening dark bands :
side minima
Light from a small source passes by the edge ofan opaque object and continues on to a screen.
DIFFRACTION PATTERNS FROM NARROW SLITS
In order to the rays reaching the screen are approximately parallel, assume that The observing screen is far from
the slit We using a converging lens to
focus the parallel rays on a nearby screen
The pattern of the screen is called a Fraunhofer diffraction pattern Fraunhofer diffraction pattern of
a single slit.
Each portion of the slit acts as a source of light waves.
Waves from the upper half of the slit interfere destructively with waves from the lower half when
The general condition for destructive interference is
Intensity distribution for a Fraunhofer diffraction pattern from a single slit of width a
EXAMPLE Light of wavelength 580 nm is incident on a slit
having a width of 0.3 mm. the viewing screen is 2 m from the slit. Find the positions of the first dark fringes and the width of the central bright fringe
The width of the central bright fringe is equal to 2y1
INTENSITY OF SINGLE-SLIT DIFFRACTION PATTERNS
Minima occur when
EXAMPLE Find the ratio of the intensities of the
secondary maxima to the intensity of the central maximum for the single-slit Fraunhofer diffraction pattern
RESOLUTION OF SINGLE SLIT
RESOLUTION OF CIRCULAR APERTURES
EXAMPLE Light of wavelength 589 nm is used to view
an object under a microscope. If the aperture of the objective has a diameter of 0.900 cm. What is the limiting angle of resolution? If it were possible to use visible light of any
wavelength, what would be the maximum limit of resolution for this microscope?
THE DIFFRACTION GRATING
A useful device for analyzing light sources Consists of a large
number of equally spaced parallel slits
Condition for interference maxima for a grating:
EXAMPLE Monochromatic light from a helium-neon laser (λ
= 632.8 nm) is incident normally on a diffraction grating containing 6000 grooves per centimeter. Find the angles at which the first and second order
maxima are observed
DIFFRACTION OF X-RAYS BY CRYSTALS
Crystalline structure of NaCl. The blue spheres represent Cl- ions The red spheres represent Na+ ions
The condition for constructive interference (maxima in the reflected beam) is
Bragg’s Law