Transcript Presentation Lesson 26 Diffraction and Interference

Lesson 26
Diffraction and Interference
Eleanor Roosevelt High School
Chin-Sung Lin
Diffraction
Christiaan Huygens
• Christiaan Huygens, a Dutch physicist ,
astronomer, and mathematician
• Telescopic studies elucidating the
nature of the rings of Saturn and the
discovery of its moon Titan
• The invention of the pendulum clock
• Studies of both optics and the
centrifugal force
Huygens’ Principle
• Light waves spreading out from a point source may be
regarded as the overlapping of tiny secondary wavelets
• Every point on any wavefront may be regarded as a new point
source of secondary waves
• Wavefronts are made up of tinier wavefronts
Huygens’ Principle
• As wavefronts spread, they appear less curved. Wave fronts
seem to form a plane very far from the original source
Huygens’ Principle
• Huygens’ principle applied to reflection
Huygens’ Principle
• Huygens’ principle applied to refraction
Huygens’ Principle
• Straight wave passing through wide opening
Huygens’ Principle
• Straight wave passing through narrow opening
Huygens’ Principle
• When the opening becomes smaller, Huygens’ idea that every
part of a wave front can be regarded as a source of new
wavelets becomes apparent
Huygens’ Principle
• Huygens’ principle applied to diffraction
Diffraction
• The bending of a wave by means other than reflection or
refraction as it passes around the edge of an obstacle
Diffraction
• When light passes through an opening that is large compared
to the wavelength of light, it casts a rather sharp shadow
• When it passes through a small opening it casts a fuzzy
shadow
Diffraction
• Single-color light passing through a razor blade creates
diffraction fringes
• In white light, the fringes merge together to create a fuzzy
blur at the edge of the shadow
Diffraction
• The amount of diffraction depends on the size of the
wavelength compared to the size of the obstruction that casts
the shadow
• The longer the wave compared to obstruction, the greater the
diffraction is
• Long waves are better at filling in shadows
Diffraction
• Long waves diffract or bend readily around buildings and
reach more places than shorter waves do
• FM radio waves have shorter wavelengths than AM waves do,
so they don’t diffract as much around buildings
• Many places have poor FM reception but clear AM stations
Diffraction
• Diffraction limits the function of optical microscope
– Size of the object = the wavelength of the light: blurred
– Size of the object < the wavelength of light: no structure
will be seen
Diffraction
• Electron microscopes are used to illuminate tiny things where
the is much less than that of an optical microscope
Interference
Interference
• When two or more waves happen at the same time, they
cross each other and produce an interference pattern
• Wave effects may be increased, decreased or neutralized
within this pattern
Young’s Interference Experiment
• British physicist, Thomas directed monochromatic light
through two closely spaced slits, fringes of brightness and
darkness were produced on a screen behind to demonstrated
wave nature of light that Huygens had previously suggested
Interference
• The bright fringes of light resulted from light waves from both
slits arriving crest to crest
• Dark areas resulted from light waves arriving trough to crest
Interference
• The path difference is the reason of the interference
Interference
• Color changes, the interference pattern changes
Interference
• Diffracting grating is produced by a multitude of closely
spaced parallel slits
• Many spectrometers use diffracting gratings rather than
prisms to disperse light into colors
Interference
• Whereas a prism separates colors of light by refraction, a
diffracting grating separates colors by interference
• Diffracting grating can be observed in peacock’s feathers, CD
discs etc.
Interference from Thin Films
• Interference fringes can be produced by the reflection of light
from two surfaces that are closely together
• When monochromatic light is shone onto two plates of glass,
one atop the other, dark and bright bands can be observed
Interference from Thin Films
• Iridescence: interference of light waves of mixed frequencies
reflected from the top and bottom of thin films, produces a
spectrum of colors
Interference from Thin Films
• Testing of precision lenses can
be a practical use of
interference fringes
• When a lens that is to be tested
is placed on a perfectly flat
piece of glass light and dark
concentric, and regularly
spaced fringes are seen
Interference from Thin Films
• Testing of precision lenses can
be a practical use of
interference fringes
• When a lens that is to be tested
is placed on a perfectly flat
piece of glass light and dark
concentric, and regularly
spaced fringes are seen
Laser Light
• Laser (Light Amplification by Stimulated Emission of Radiation)
Laser Light
• Light emitted by a common lamp is incoherent (many phases
of vibration)
• Interference within a beam of incoherent light is rampant and
a beam spreads out after a short distance, becoming wider
and less intense
Laser Light
• A filtered monochromatic beam is still incoherent since the
waves are out of phase and interfere with one another. The
slightest differences in their directions result with increased
distance
Laser Light
• Laser produces coherent light that has the same frequency,
phase and direction. There is no interference of waves within
the beam. Only a beam of coherent light will not spread and
diffuse
Laser Light
• A laser is a device that emits light through a process of optical
amplification based on the stimulated emission of photons
Laser Light
• Laser is on
Laser Light
• Within a laser, light wave emitted from one atom stimulates
the emission of light from a neighboring atom so that the
crests of each wave coincide
Laser Light
• These waves stimulate the emission of others in cascade
fashion and a beam of coherent light is produced
Hologram
• Hologram is a 3-D version of a photograph that contains the
entire message in every portion of its surface. Light diffracted
from these fringes produces an image that is extremely
realistic
Hologram
Hologram
• Holograms are produced by interference between two laser
light beams on photographic film
• The object scatters the light in all directions including onto the
film the film is also illuminated by means of another diverged
beam from the same laser in order to produce interference
• Interference between the reference beam and light reflected
from the different points on the object produces a pattern of
microscopic fringes on the film
• Light from nearer parts of the object travels shorter paths
than light from farther parts of the object. The different
distances traveled will produce slightly different interference
patterns with reference beam. In this way, information about
the depth of an object is recorded
Hologram
• When light falls on a hologram, it is diffracted by the fringed
pattern to produce wave fronts identical in form to the
original wave fronts reflected by the object. The diffracted
wave fronts produce the same effect as the original reflected
wave fronts
Hologram
• If holograms are made using short-wavelength light and
viewed with light of a longer wavelength, the resulting image
is magnified in the same proportion as the wavelengths
• Holograms made with X-rays would be magnified 1000x when
viewed with visible light and appropriate viewing
arrangements
The End