Today: Sect' 28'3 Thinfilm interference

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Today Sect. 28.3 Thin-film interference
Sect. 28.6 Diffraction gratings Wednesday
Chapter 29 Relativity!
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A train of waves of fixed frequency is traveling
along a thin, light cord toward the juncture to
a heavy, thick cord. Both cords are stretched
taut and under the same tension.
The waves that travel across the heavy cord will
have
1) greater speed than along the lighter cord.2)
the same speed as along the lighter cord. 3)
smaller speed than along the lighter cord.
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A train of waves of fixed frequency is traveling
along a thin, light cord toward the juncture to
a heavy, thick cord. Both cords are stretched
taut and under the same tension.
The waves traveling across the heavy cord have
1) larger wavelength than along the lighter
cord.2) the same wavelength as along the lighter
cord. 3) smaller wavelength than along the
lighter cord.
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Direction of electric magnetic fields invert,
undergoing a 180o phase shift
air
glass
No phase shift! Preserves E, B field directions
upon reflection.
air
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Direction of electric magnetic fields invert,
undergoing a 180o phase shift
air
water n1.3
Direction of electric magnetic fields invert,
undergoing a 180o phase shift
glass n1.5
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Direction of electric magnetic fields invert,
undergoing a 180o phase shift
air n1.00
oil n1.38
No phase shift! Preserves E, B field directions
upon reflection.
water n1.33
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Another way to get Interference

• Thin oil film on water
• Part of the incoming light is reflected off the
  top surface (point A), part at the lower surface
  (point B).
• Light traveling through oil travels extra
  distance (from A to B to C).
• If this distance is l, 2l, 3l, 4l,
• constructive interference!
• If this distance is l/2, 3/2l, 5/2l,
• destructive interference!

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If 2t exactly 1 ? or any whole number multiple
of a wavelength 2t m? the two waves would
combine
air
t
oil film n1.38
water n1.33
a. in phase and constructively interfere b.
out of phase and destructively interfere.
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If 2t exactly 1/2 ? or any whole number
multiple of a wavelength 2t (m1/2)? the two
waves would combine
air
t
soap film n1.25
water n1.33
a. in phase and constructively interfere b.
out of phase and destructively interfere.
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Non-reflecting lenses
The blue-purple tint of camera and binocular
lenses is due to a think coating intended to make
them invisible
or at least reduce reflections
not by making them less shiny but by
the destructive interference of the reflected
light!
Air, nO? 1.00
180o phase on each reflection
Thin film, n1
Glass lens, n2
destructive interference
constructive interference
but this of course works exactly for only 1
precise wavelength!
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A glass (n1.60) lens is coated with a thin film
of fluoride (MgF2, n1.38). What minimum film
thickness makes ?550 nm light non-reflecting?
t?/4
180o phase change for each reflection
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Diffraction Gratings
Surface machine scored with many parallel
scratches
5000 lines /cm
Spacing d 1 cm/5000 2?10-4 cm
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dsin? m?
m 0, ?1, ?2, ?3,
m 0
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dsin? m?
sin? m?/d
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3) smaller speed than along the lighter cord.
3) smaller wavelength than along the lighter cord.
b. out of phase and destructively interfere.
b. out of phase and destructively interfere.