Diffraction and Interference

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Diffraction and Interference

• Diffraction
• Huygens Principle
• Diffraction Lab

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Light

• Has wave properties.
• Can diffract.
• Can constructively or destructively interfere.

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Wave Fronts

• Lines that are perpendicular to the motion of the
  wave.
• Indicate the location of the crests in the waves
  that are traveling together.

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Huygens Principle

• Wave fronts are made up of tinier wave fronts.
• Every point on any wave front is a new source for
  a secondary wave front.

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Huygens Principle

• You can explain reflection and refraction using
  Huygens Principle.

http//www.microscopy.fsu.edu/primer/java/reflecti
on/huygens/
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Huygens Principle

• As the straight waves passed through a narrow
  hole, they spread out in a circular pattern.
• Giving proof to the fact that every point on a
  wave front is a new source for a new set of
  wavelets.

http//www.ngsir.netfirms.com/englishhtm/Diffracti
on.htm
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Diffraction

• Any bending of a wave around an obstacle or edges
  of an opening by means other than reflection or
  refraction.

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Diffraction
Demo

• The amount of diffraction (bending) depends on
  the size of the wavelength compared with the size
  of the obstruction.
• The longer the wavelength is compared to the
  obstruction, the greater the diffraction.

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Is Diffraction a Good Thing?

• Why would we ever want waves to bend past an
  obstruction?

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Is Diffraction a Good Thing?

• Long AM radio waves can diffract around hills and
  buildings and can be received better in more
  places than short waves that dont diffract as
  much.

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Is Diffraction a Good Thing?

• Diffraction is bad when we want to see very small
  objects with microscopes.
• If the size of the small object is the same as
  the wavelength of light, the image will be
  blurred by diffraction.

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Interference
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Youngs Interference Experiment

• 1801, Thomas Young discovered that when light of
  a single color (monochromatic) was directed
  through two closely spaced pinholes, fringes of
  brightness and darkness were produced on a screen.

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Youngs Interference Experiment

• Bright fringes constructive interference
• Waves arrive at the screen in phase
• Dark fringes destructive interference
• Waves arrive at the screen out of phase

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Diffraction Grating

• A series of closely spaced parallel slits or
  grooves that are used to separate colors of light
  by interference.
• Different colors have different wavelengths and
  diffract at different rates.
• So they constructively interfere at different
  places.

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Single-Color Interference from Thin Films

• Interference fringes can be produced by the
  reflection of light from two surfaces that are
  very close together.
• If you shine a single-color (monochromatic) light
  onto stacked (with an air wedge) plates of glass,
  youll see dark and bright bands.

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Single-Color Interference from Thin Films

• The reason for the dark/bright bands is that
  reflected light from the top plate interferes
  destructively/constructively with light reflected
  from the bottom plate.

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Single-Color Interference from Thin Films

• Practical uses would be to test the precision of
  lenses.
• Straight/round fringes perfectly flat/round
  glass
• Irregular fringes irregular surface

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Iridescence from Thin Films
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Iridescence from Thin Films

• Iridescence The phenomenon whereby interference
  of light waves of mixed frequencies reflected
  from the top and bottom of thin films produces a
  spectrum of colors.

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Iridescence From Thin Films

• A thin film, such as a soap bubble or oil on
  water, has two closely spaced surfaces.
• Light that reflects from one surface may cancel
  light of a certain frequency that reflects from
  the other surface.

http//webphysics.davidson.edu/physlet_resources/b
u_semester2/c26_thinfilm.html
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Iridescence From Thin Films

• If the film is illuminated with white light and
  the light that reflects to your eye has blue
  cancelled due to the reflected light from the
  other surface, what color will you see?

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Iridescence From Thin Films

• If the film is illuminated with white light and
  the light that reflects to your eye has blue
  cancelled due to the reflected light from the
  other surface, what color will you see?
• The complementary color, yellow!

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Iridescence from Thin Films

• Same principles as Single-Color Interference
• The shapes of the fringes for both are made by
  the differences in thickness of the materials.

Except we are using light of mixed frequencies
and our fringes are made of different colors.
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Incoherent Light

• Light emitted by a common lamp is incoherent. It
  has many phases of vibration as well as many
  frequencies.
• Incoherent light spreads out after a short
  distance and loses intensity.

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Coherent Light

• A beam of light that has the same frequency,
  wavelength, phase, and direction is called
  coherent.
• There is no interference of waves within the beam
  and the beam will not spread out and diffuse.

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Laser Light

• Laser light is coherent.
• LASER Light Amplification by Stimulated
  Emission of Radiation

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The Laser

• In a laser, a light wave emitted from one atom
  stimulates the emission of light from a
  neighboring atom so that the crests of each wave
  coincide. Thus a coherent beam.

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The Hologram

• The three-dimensional version of a photograph
  produced by interference patterns of laser beams.

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The Hologram

• The interference of the laser beams produces
  fringe patterns on the photographic plate that
  record the depth of the surface of an object.

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The Hologram

• The fringe pattern of a hologram diffracts light
  to produce wave fronts identical to the wave
  fronts given by the object.

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The Hologram
So you see the 3-D image due to the way the
hologram diffracts light and the way this
diffracted light constructively and destructively
interfere. In this way, holograms are like
diffraction gratings.
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The Hologram

• Every part of the hologram receives and records
  light from the entire object, so you can cut a
  hologram in half and still be able to view the
  whole image.

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The Hologram

• You can magnify the image of a hologram by
  looking at it with light that has a longer
  wavelength than which it was made.