Wave Nature of Light

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Wave Nature of Light

• How do you cook an ant on the sidewalk
• Get a magnifying glass and focus the suns rays
  on the ant and the ant gets hot!
• If the ant heats up the light must have supplied
  the energy!
• How did it do this

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Wave Nature of Light

• We saw how Maxwells Equations predicted that
  alternating electric field produced a magnetic
  field and an alternating magnetic field produced
  an electric field
• The theory predicted electromagnetic waves!
• These waves must transport the energy!

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Wave Nature of Light

• Last fall we saw how water waves could carry
  energy
• They make a leaf bob up and down on the surface
  of a lake when waves roll by
• So we know that waves can carry energy
• We just finished looking at a ray model of
  light to explain reflection and refraction

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Wave Nature of Light

• Why would we think that light is a wave
• DIFFRACTION!!!
• Remember we saw that water waves diffracted
  around corners when the wavelengths were
  comparable to the size of the obstacle
• To see light diffraction we have to have some
  small obstacles

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Wave Nature of Light

• Back in the 1600s Huygens developed an approach
  that is very helpful
• Every point on a wave front can be considered as
  a source of tiny wavelets that spread out in the
  forward direction at the speed of the wave
  itself. The new wave front is the envelope of
  all the wavelets - the tangent to all of them.

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Wave Nature of Light
Huygens Principle shows how the wave front moves
from position AB to position CD. We just create a
lot of new little sources and let each one
generate a new wave. The combination forms a new
wave front!
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Wave Nature of Light

• See how the wavelets at the edges let the light
  bend around corners!!!
• As the slit gets smaller the bending is more
  obvious.

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The Wave Nature of Light

• The law of refraction is explained by Huygens
  Principle

The little wavelets move slower in medium 2 than
in medium 1. Doing the tangent shows how the
wave fronts bend.
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The Wave Nature of Light
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The Wave Nature of Light

• The second big argument for light being a wave is
  the phenomenon of interference
• We looked at wave interference last fall
• We decided that you just add the amplitudes of
  the two waves that occupy the same region of space

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Youngs Double Slit
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Youngs Double Slit
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Youngs Double Slit
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Youngs Double Slit
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Youngs Double Slit
The sources have to be coherent that is they
have the same phase relationship. The wave peaks
must start out in sync at the two slits.
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Dispersion
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Dispersion