Dr. Quantum
General Physics 2
Light as a Wave
1
The Nature of Light
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When studying geometric optics, we used a ray
model to describe the behavior of light.
A wave model of light is necessary to describe
phenomena such as:
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interference
diffraction
A particle model of light is necessary to describe
phenomena observed in modern physics, for example,
the interaction between light and atoms. We’ll get
back to this later...
General Physics 2
Light as a Wave
2
Wave Nature of Light
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Christian Huygens (16291695)
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contemporary of Newton
developed wave theory of light
Huygen’s Principle
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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 tangent to all of them
General Physics 2
Light as a Wave
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Huygen’s Principle
General Physics 2
Light as a Wave
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Diffraction
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Huygen’s Principle is useful for understanding
diffraction - the bending of waves behind
obstacles into the shadow region
General Physics 2
Light as a Wave
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Interference
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Thomas Young (1773-1829)
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definitively (at least temporarily)
demonstrates wave nature of
light
Young’s Double-Slit
Experiment
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coherent light passes through 2
slits, S1 and S2
light from S1 and S2 then
interferes and pattern of dark
and light spots is observed on
the screen
General Physics 2
Light as a Wave
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Interference
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Constructive interference occurs when
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Destructive interference occurs when
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d sin = m  , m = 0,1,2,...
m = order
d sin = (m + 1/2)  , m = 0,1,2,...
Source must be coherent
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waves at S1 and S2 are in-phase
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Light as a Wave
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what you see on the screen:
General Physics 2
Light as a Wave
8
Think-Pair-Share
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Monochromatic light falling on two slits 0.016
mm apart produces the fifth-order fringe at an
8.8 degree angle. What is the wavelength of
the light used?
General Physics 2
Light as a Wave
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Pair Problem
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Light of wavelength 680 nm falls on two slits
and produces an interference pattern in which
the fourth-order fringe is 38 mm from the
central fringe on a screen 2.0 m away. What
is the separation of the two slits?
(Hint: tan =  for small angles, and angles
must be in radians!)
General Physics 2
Light as a Wave
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Visible Spectrum
General Physics 2
Light as a Wave
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Dispersion
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Index of refraction varies with wavelength of light
As a result, white light is separated into component
colors by a prism or by water (rainbow)
General Physics 2
Light as a Wave
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Dispersion & Rainbow
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red is bent the least
red light reaches observer’s eye from higher water droplets
violet is bent the most
violet light reaches observer’s eye from lower water droplets
General Physics 2
Light as a Wave
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Diffraction by a Disk
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Diffracted light interferes constructively at center of
shadow
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requires a point source of monochromatic light (e.g. laser)
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Light as a Wave
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Diffraction by a Single Slit
position of minima
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D sin  = m 
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m = 1, 2, 3, ...
for m=1, theta gives 1/2 width
of central maximum
General Physics 2
Motivation for making large
diameter telescopes
Light as a Wave
15
Diffraction Grating
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a large number of
equally spaced parallel
slits
same relation as
double-slit
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d sin  = m 
m = 0, 1, 2, ...
produces sharper and
narrower interference
patterns that double
slit
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Light as a Wave
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Diffraction Grating
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double slit versus
diffraction grating
General Physics 2
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for multi-wavelength
light
Light as a Wave
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Interference by Thin Films
Produces rings of
constructive/destructive
interference
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Light as a Wave
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Thin-Film Interference: Examples
Oil on water
Soap bubbles
Beetles
Butterflies
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Light as a Wave
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Find the diffraction grating spacing
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Align the laser and the diffraction grating
Measure the length and height of peaks to
determine the angle
Use the diffraction equation to calculate the
diffraction grating spacing.
The laser wavelength is 630 nm
General Physics 2
Light as a Wave
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PhET Conceptual Question
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Use PhET simulation to answer the following questions
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http://phet.colorado.edu/en/simulation/wave-interference
What happens to the interference pattern if the wavelength of
light is increased from 500 nm to 700 nm?
What happens instead if the wavelength stays at 500 nm but the
slits are moved farther apart?
General Physics 2
Light as a Wave
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Emission Tubes – real and PhET
Look at several emission tubes using diffraction
Element gratings & sketch spectrum
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Foundation of spectroscopy, a technique used in
numerous scientific applications
Compare with PhET simulation
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http://phet.colorado.edu/en/simulation/discharge-lamps
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Light as a Wave
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