Wave packet etc
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Plane Waves
• a plane wave is an ideal wave withsingle wavenumber k. The wave fronts(surfaces of constant phase) of a plane
wave are innite parallel planes ofconstant peak-to-peak amplitude normalto the phase velocit vector.
• !f a wave is of innite e"tent onl then itwill propagate as a plane wave. !n mostpractical cases waves are appro"imatelplane waves in a locali#ed region of space.
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Phase $elocit of a PlaneWave
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Plane Wave
Equation of a Plane% plane in &d-space has the e'uation
ax + by + cz = d,where at least one of the numbers a b c must be
non#ero.
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Wave packet in bothposition and momentum space
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$elocit of a Wave Packet
• !f the position of the wave packetchanges with time then the rate atwhich the ma"imum point moves is a
reasonabl good measure of thevelocit of the wave packet
∫ ∞
∞−
−= dk ek at x wt kxi )()(),(ψ
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%ppro"imations•
!n order for the momentum of the particleto be reasonabl well dened
--- et us assume that the harmonic wavespresent in ψ ("t) have wave vectors lingwithin a small range centered around avalue *k+
k p =
)(
)()(
)()(
k k
k k k d
d k
k k dk
d
k k k
==
−
+=
−
+= =
ω ω
ω ω ω
ω
ω ω
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%t the ,a"imum of Wave packet at ",%
0=
= MAX x xdx
d ψ
∫ ∞
∞−
−= dk ek at x wt kxi )()(),(ψ
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%t the ,a"imum of Wave packet at ",%
∫ ∞
∞−
−= dk ek at x wt kxi )()(),(ψ
∫ =−0dk eik k a
t kxi MAX )(
))(( ω
0=
= MAX x xdx
d ψ
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urther di/erentiating wrt to time
∫ =−0dk eik k a
t kxi MAX
)())((
ω
( )[ ]∫ =− −0dk e xk iik k a
t kxi
MAX
MAX )())((
ω
ω
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0sing the %ppro"imation
( )[ ]∫ =− −0dk e xk iik k a
t kxi
MAX
MAX )())((
ω
ω
)()( k k k d
d k −
+= ω
ω ω
∫ ∫ =
−−
− −−
022
dk keik ak k d
d dk eik k a
k d
wd x
t kxit kxi
MAX
MAX MAX )()())(( ))((
ω ω ω
ω
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0sing the %ppro"imation
∫ ∫ =
−−
− −− 022 dk keik ak
k d
d dk eik k a
k d
wd x t kxit kxi MAX
MAX MAX )()( ))(( ))(( ω ω ω
ω
∫ =−0dk eik k a
t kxi MAX )(
))(( ω
∫ =
− −
02
))(()(dk eik k a
k d
wd x
t kxi
MAX
MAX ω
0=
= MAX x xdx
d ψ
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0sing the %ppro"imation
∫ =
− −0
2 ))((
)(dk eik k a
k d
wd x
t kxi
MAX
MAX ω
02
2
≠
= MAX x x
dx
d ψ
The term in the above integral ison!zero"
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$elocit of the Peak
PacketWavetheof VelocityGroupcalledisG
G
MAX
MAX
MAX
V
k d
wd V
k d
wd
dt
dx
k d
wd
dt
dx
k d
wd x
=
=
=
−
=
−
0
0
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Phsical 1ignicance of $2
• The $elocit of a free particle (bothnon-relativistic and relativistic) canbe written as
dp
dE v =
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Phsical 1ignicance of $2
• The $elocit of a particle (both non-relativistic and relativistic) can bewritten as
dp
dE v particle =
k p & E == ω with
G particle V dk
d
dp
dE v ~
ω ==
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The Particle Picture
G particle V dk
d
dp
dE v ~ ω ==
% small wave packet consisting of small band of3e 4roglie waves moves somewhat like a
classical free particle.
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Wave-Particle 3ualit
G particle V dk
d
dp
dE v ~ ω ==
% small wave packet consisting of small band of3e 4roglie waves moves somewhat like a
classical free particle.
The appro"imation is valid onl to the e"tent that onecan ignore the fundamental limitations placed b thewave packet picture on the accurac with which boththe position and momentum of a particle can be