Diffraction and Interference

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Diffraction and Interference

• Interference
• Youngs Interference Experiment
• Diffraction Grating
• Single-Color Interference
• Iridescence

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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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CD 31-1