Wave Nature of Light: Interference and Diffraction

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PH103
Wave Nature of Light Interference and Diffraction
Dr. James van Howe Lecture 9
March 31, 2008
Water wave diffraction Sitges, Barcelona
Photo Barcelona Field Studies Center
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Recall EM plane wave looks like
, wavelength (spatial period)
distance
We are looking at the wave in a snapshot of time,
I can write it just as a function of space as
shown below.
D
Electric Field, E
distance, z
Electric Field
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Time dependence
, wavelength (spatial period)
x
distance
If we sit on one crest (like at the smiley face)
we see that the amplitude also goes up and down
like a sine wave as a function of time.
D
Electric Field, E
time, t
Electric Field
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Constructive Interference
Source 1
Source 1
Two beams of light at the same frequency and same
amplitude, in phase (p, 2p, 4p radians out of
phase even multiples of p), at the same amplitude
position, z
Final Electric Field
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Destructive Interference
Source 1
Source 1
Two beams of light at the same frequency and same
amplitude, but out of phase (p, 3p, 5p radians
out of phase odd multiples of p),
position, z
Electric Field, E
Final Electric Field
Same thing happens in noise canceling headphones
and auto mufflers
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Huygens-Fresnel Principle
Every point on a wavefront serves as a secondary
source of point sources

Plane Wave, Arial View
Infinite amount of point sources plane wave
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Waves will diffract around a barrier
Bending
Many point sources, but not infinite
Plane Wave
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Diffraction by a Single Slit
Case 1
Infinite point sources fit inside aperture
Shadow
plane wave out
Bright
Like light going through a window
Shadow
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Diffraction by a Single Slit
Many point sources, but not infinite
spreading
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Diffraction by a Single Slit
Slit so small only one point source fits
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Double Slit Diffraction
like two point sources
could be two speakers, small slits for light, two
stones on pond
Lines of constructive interference
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Poissons Spot
If obstruction size is on the order of
wavelength diffraction
Bright spot behind obstacle
Demo Video ripple tank, acoustic Poisson
Demo optics card
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Interference The property of waves to add up
(superpose) constructively or destructively
In order to have good interference, the waves
have to be coherent
Coherent source when any two waves emitted from
that source have a fixed relationship in
time/space
Wave 1
Good coherence Good interference
Wave 2
Example laser
Wave 1
Wave 2
Bad coherence Bad interference
Example sunlight
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Homework
Ch24 15, 25