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