10/6/2011
Regarding electrons, what is the difference between a conductor and an insulator? Conductor has free electrons which can easily move.
What field or fields, if any, compose an electromagnetic wave? Electric and Magnetic.
Microwaves get their name from their wavelength, which is about 1/1,000,000 meter. What is the frequency of such a light wave? You now know this was a lie, but given the bogus wavelength, the answer would be f = c/λ = 3x108 m/s / 1x10‐6 m = 3x10‐14 /s = 3x1014 Hz.
Do microwaves oscillate at a higher or low frequency than visible light? Lower.
If a pulse of the waves in problem # 3 lasts 10‐10 sec, how many wavelengths are in it? N = f x Δt = 3x1014 Hz x 10‐10 s = 3x104 .
How long in space is the wave train in problem #5? L = λ x N = 10‐6 m x 3x104 = 3x10‐2m = 0.03 m.
What physically changes in a light wave which enters a block of clear glass? Speed and wavelength; NOT frequency.
Light from the sun falls on a linear Polaroid, and then passes through another linear Polaroid rotated 90° to the first Polaroid. How much of the original sunlight gets through both Polaroid’s? None—Zero.
Two light waves (red and blue) bend on going through a slit and then again on going through a prism of glass. Is it possible for them to make a bright spot on a screen in the same place, or is this impossible? Explain. Since the beam experiences two effects which have opposite dependences on wavelength, it is possible. As they go thru the slit, the red would bend more than the blue, but on going thru the prism, the blue would bend more than the red, making it possible, by proper positing of the slit, the prism, and the screen to bring the different color beams back together on a screen.
What is Brewster’s angle for light reflection off a clear lake? tanӨ = n = 1.33 so Ө = Inverse (tan 1.33) = 53°
Which pole of the Earth’s magnet is near the north geographic pole? South.
Why does putting two polarizers oriented at right angles to each other in front of each slit in Young’s interference experiment prevent the interference pattern from being seen? Each slit then passes light of opposite polarization, which means the light from each slit is coming from different atoms. Only a single photon, coming from one atom, can interfere with itself—only then is there coherence.
When electrical charges move in solid substances they are usually electrons?
Polarization by Selective Absorption
• The most common technique for polarizing light
• Uses a material that transmits waves whose electric field vectors in the plane are parallel to a certain direction and absorbs waves whose electric field vectors are perpendicular to that direction
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Single Slit Diffraction
• The results of the single slit cannot be explained by geometric optics
– Geometric optics would say that light rays traveling in straight lines should cast a sharp image of the slit on the screen
E. H. Land discovered a material that polarizes light through
selective absorption
 He called the material Polaroid
 The molecules readily absorb light whose electric field vector is
parallel to their lengths and transmit light whose electric field
vector is perpendicular to their lengths
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When an unpolarized light beam is reflected from a surface, the
reflected light may be completely or partially polarized
n
sin  p
cos  p
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 tan p
θp is called Brewster’s Angle = 100% polarization
All the waves that originate at the slit are
in phase
Wave 1 travels farther than wave 3 by an
amount equal to the path difference
(a/2) sin θ
If this path difference is exactly half of a
wavelength, the two waves cancel each
other and destructive interference results
Light and Dark Fringes from 2 slits
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Bright fringe, produced by constructive interference,
path difference must be either zero or some integral
multiple of the wavelength λ
δ = d sin θbright = m λ
 m = 0, ±1, ±2, …
When destructive interference occurs, a dark fringe is
observed
This needs a path difference of an odd half wavelength
δ = d sin θdark = (m + ½) λ
 m = 0, ±1, ±2, …
Position of Fringes from 2 slits
• The positions of the fringes can be measured vertically from the zeroth order maximum
• y = L tan θ  L sin θ
Assumptions: L>>d and d>>λ
• Approximation
– θ is small so tan θ  sin θ
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For bright fringes
y bright 
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L
For dark fringes
ydark 
L 
d
m
1
m 
d 
2
m  0,  1,  2 
m  0,  1,  2 
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10/6/2011
• Thin Film Interference
• Ray 1 undergoes a phase change of 180° with respect to the incident ray
• Ray 2 undergoes no phase change with respect to the incident wave
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Ray 2 travels extra distance 2t before the waves recombine
Constructive interference
 2nt = (m + ½ ) λ
For destruction interference
 2 n t = m λ
m = 0, 1, 2 …
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Include two effects in thin film interference
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Path length and possible Phase change
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