Chapter 28
SUPERPOSITION AND INTERFERENCE:
For (b), which peak is which?
They all look the same.
1.
Addition of ordinates – when adding two waves their ycoordinates (amplitudes) add together
at every point to produce a new wave.
2.
IN PHASE
means that the peaks and troughs of the two waves “line up” with each other.
3.
When two waves combine in phase, the resulting wave is the maximum possible.
This is
called
CONSTRUCTIVE INTERFERENCE.
For light waves, this would be a bright spot
or line.
4.
When two waves are 90° out of phase (peak to trough) the result is a null wave, or no wave at
all.
This is called
DESTRUCTIVE INTERFERENCE
.
This would appear as a dark spot
or line.
The above rules apply to light
that is both
MONOCHROMATIC
(same color/frequency) and
COHERENT
(the phase different between the two waves remains constant).
To determine whether there will be constructive or destructive interference at some point, it’s
necessary to find the
phase difference
(where is the peak of one wave compared to the other
wave?)between the two waves at that point.
1.
Find the length difference that the two waves travel, called the
path length difference.
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2.
If this length is a multiple of whole wavelenths, we get
constructive
interference.
3.
If this length is an odd halfwavelength, we get
destructive
interference.
2
1
1
2
1
2
m
constructive interference
(m
)
destructive interference
4.
•• A person driving at 17 m/s crosses the line connecting two radio transmitters at right
angles, as shown in Figure 28–31. The transmitters emit identical signals in phase with each
other, which the driver receives on the car radio. When the car is at point A the radio picks up a
maximum net signal. (a) What is the longest possible wavelength of the radio waves? (b) How
long after the car passes point A does the radio experience a minimum in the net signal? Assume
that the wavelength has the value found in part (a).
(a)
(450m
150m)
300m
m
is maximum when m is minimum
1
2
2
2
2
1
2
1
2
2
2
2
1
1
2
2
y
vt
y
t
We want y such that
(m
)
v
y
(450m)
y
(150m)
2
(
)
2(
)
(
)
4
2
YOUNG’S DOUBLESLIT EXPERIMENT: (17731829) – helped to decipher the Rosetta
Stone.
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 Fall '06
 Alexandrakis
 Physics, Diffraction, The Elegant Universe, Wavelength, Tan, θcf

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