9/21/2010 Phase Changes Due To Reflection HITT RF Remote Login Procedure:

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9/21/2010
It is against the honor code to “click” for
someone else-violators will loose all clicker pts.
HITT RF Remote Login Procedure:
Phase Changes Due To Reflection

The radio channel number for this room is “09” (zero, nine).
It is STRONGLY recommended to login your remote for every class just
to be sure it is on the correct radio channel and working before class.
1.
2.
3.
4.
PRESS AND HOLD THE DOWN ARROW KEY until
the GREEN light on the remote turns RED.
PRESS THE “0” KEY and you will see the RED light
flash GREEN.
PRESS THE “9” KEY and you will see the RED light
flash GREEN.
PRESS AND RELEASE THE DOWN ARROW KEY
again and you will see the red light search for the
receiver, if it BLINKS GREEN MULTIPLE TIMES you
are logged in.
Analogous to a reflected pulse on a string
Interference in Practice
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An electromagnetic wave undergoes a phase
change of 180° upon reflection from a medium
of higher index of refraction than the one in
which it was traveling
Next lab will use Young’s Double Slit
Experiment to measuring wavelength of
the light
Lloyd’s Mirror
Thin Films
Newton’s Rings
DVDs
Phase Change going from high to low n

There is no phase change when the wave is reflected
from a boundary leading to a medium of lower index of
refraction
Analogous to a pulse in a string
reflecting from a free support
Interference in Thin Films
Lloyd’s Mirror

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Another arrangement for producing an interference
pattern with a single light source
Wave reachs point P either by a direct path or by
reflection
The reflected ray can be treated as a ray
from the source S’ behind the mirror
The positions of the
dark and bright
fringes are reversed
relative to pattern of
two real sources
This is because there
is a 180° phase
change produced by
the reflection


Examples are soap bubbles and oil on water
Due to interference of waves reflected from
both surfaces of the film
Two facts-An electromagnetic wave traveling:
 from low index medium n1 toward higher
index medium n2 undergoes a 180° phase
change on reflection
 There is no phase change in the reflected
wave if n2 < n1
 The wavelength of light λn in a medium with
index of refraction n is λn = λ/n
(λ is the wavelength of light in vacuum)

1
9/21/2010
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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
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 …

Example of Thin Film Interference Problem
Coating on a solar cell
 Two phase changes
 Three indices of refraction
Quick Quiz
Which formula applies for
constructive interference?
A. 2nt = (m + ½) 
B. 2nt = m 
Include two effects in thin film interference
 Path length and possible Phase change
Newton’s Rings

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Another method for viewing interference is to
place a plano-convex lens on top of a flat glass
surface
The air film between the glass surfaces varies
in thickness from zero at the point of contact to
some thickness t
A pattern of light and dark rings is observed
 This rings are called Newton’s Rings
 The particle model of light could not explain
the origin of the rings
Newton’s Rings can be used to test optical
lenses
Problem Solving Strategy with Thin Films
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Identify the thin film causing the interference
Determine the indices of refraction in the film
and the media on either side of it
Determine the number of phase reversals:
zero, one or two
Equation
1 phase
reversal
0 or 2 phase
reversals
2nt = (m + ½) 
constructive
destructive
destructive
constructive
2nt = m 
2
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