Wave Nature of Light

1
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?

2
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!

3
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

4
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

5
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.

6
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!
7
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.

8
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.
9
The Wave Nature of Light
10
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

11
Youngs Double Slit
12
Youngs Double Slit
13
Youngs Double Slit
14
Youngs Double Slit
15
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.
16
Dispersion
17
Dispersion