announcements 10/8/10 prayer exam: last day = tomorrow! a. a.correction to syllabus: on saturdays,...

Announcements 10/8/10 Prayer Exam: last day = tomorrow!

a. Correction to syllabus: on Saturdays, the Testing Center gives out last exam at 3 pm, closes at 4 pm.

Homework problem changes: some extra credit, one moved. See email.

Lab 3 starting tomorrow: it’s a computer simulation. See class website.

a. If we don’t finish the relevant discussion today (which seems likely), you probably should wait until after Monday’s class before starting the lab.

Quick writing assignment while you wait: Ralph is still not quite grasping this… he asks you, “How are complex exponentials related to waves on a string?” What should you tell him to help him understand? (Please actually write down your answer.)

Thought Question Which of these are the same?

(1) A cos(kx – t)(2) A cos(kx + t)(3) A cos(–kx – t)

a. (1) and (2)b. (1) and (3)c. (2) and (3)d. (1), (2), and (3)

Which should we use for a left-moving wave: (2) or (3)?

a. Convention: Usually use #3, Aei(-kx-t)

b. Reasons: (1) All terms will have same e-it factor. (2) The sign of the number multiplying x then indicates the direction the wave is traveling.

ˆk k i

Reading Quiz Which of the following was not a major

topic of the reading assignment?a. Dispersionb. Fourier transformsc. Reflectiond. Transmission

Reflection/transmission at boundaries: The setup

Why are k and the same for I and R? (both labeled k1 and 1) “The Rules” (aka “boundary conditions”)

a. At boundary: f1 = f2

b. At boundary: df1/dx = df2/dx

Region 1: light string Region 2: heavier string

in-going wave transmitted wave

reflected wave

1 1( )i k x tIA e

1 1( )i k x tRA e

2 2( )i k x tTA e

1 1 1 1( ) ( )1

i k x t i k x tI Rf A e A e 2 2( )

2i k x t

Tf A e

Goal: How much of wave is transmitted and reflected? (assume k’s and ’s are known)

x = 0

1 1 1 1 1cos( ) cos( )I I R Rf A k x t A k x t 2 2 2cos( )T Tf A k x t

Boundaries: The math

1 1 1 1 2 2( 0 ) ( 0 ) ( 0 )i k t i k t i k tI R TA e A e A e

2 2( )2

i k x tTf A e

x = 0

1 20 0B.C.1:

x xf f

1 1 2i t i t i tI R TA e A e A e

I R TA A A and 1 2

1 1 1 1( ) ( )1

i k x t i k x tI Rf A e A e

Goal: How much of wave is transmitted and reflected?


1 1 2( ) ( ) ( )1 1 2

0 0

i k x t i k x t i k x tI R T

x xik A e ik A e ik A e

2( )2

i k x tTf A e

x = 0

1 2

0 0

B.C.2:x x

df df

dx dx

1 1 2i t i t i t

I R Tik A e ik A e ik A e

1 1 2I R Tk A k A k A

1 1( ) ( )1

i k x t i k x tI Rf A e A e



Like: and

How do you solve?

x = 0

1 1 2I R Tk A k A k A I R TA A A


x y z 3 3 5x y z

2 equations, 3 unknowns??

Can’t get x, y, or z, but can get ratios!y = -0.25 x z = 0.75 x

Boundaries: The results

Recall v = /k, and is the same for region 1 and region 2. So k ~ 1/v

Can write results like this:

x = 0

1 2

1 2

R

I

A k kr

k kA


1

1 2

2T

I

A kt

k kA

2 1

1 2

R

I

A v vr

v vA

2

1 2

2T

I

A vt

v vA

“reflection coefficient” “transmission coefficient”

The results….

Special Cases

Do we ever have a phase shift? a. If so, when? And what is it?

What if v2 = 0? a. When would that occur?

What if v2 = v1? a. When would that occur?

x = 0

2 1

1 2

R

I

A v vr

v vA

2

1 2

2T

I

A vt

v vA

The results….

Power

Recall: (A = amplitude)

Region 1: and v are same… so P ~ A2

Region 2: and v are different… more complicated…but energy is conserved, so easy way is:

x = 0

2 21

2P A v

2R

I

PR r

P

21T

I

PT r

P

r,t = ratio of amplitudesR,T = ratio of power/energy

Dispersion A dispersive medium: velocity is different for

different frequenciesa. Any real-world examples?

Why do we care? a. Real waves are often not shaped like sine

waves.– Non sine-wave shapes are made up of

combinations of sine waves at different frequencies.

b. Real waves are not infinite in space or in time.– Finite waves are also made up of combinations

of sine waves at different frequencies.Focus on (b) for now… (a) is the main topic of the “Fourier” lectures of next week.

Wave packets Adding cosines together with Mathematica, “sum of

cosines.nb”

http://www.physics.byu.edu/faculty/colton/courses/phy123-fall10/lectures/lecture%2017%20-%20sum%20of%20cosines.nb

What did we learn?a. To localize a wave in space, you need lots of frequenciesb. To remove neighboring localized waves, you need those

frequencies to spaced close to each other. (infinitely close, really)




Back to Dispersion What happens if a wave pulse is sent through a

dispersive medium? Nondispersive? Dispersive wave example:

a. s(x,t) = cos(x-4t) + cos(2 (x-5t)) – What is “v”?– What is v for =4? What is v for =10?

What does that wave look like as time progresses? (Mathematica “dispersion of two cosines.nb”; on website. And next slide.)

Mathematica

0.7 seconds 1.3 seconds

0.1 seconds

Femtosecond Laser PulseFemtosecond Laser Pulse

Et=0=sin(10 x)*exp(-x^2) Power Spectrum

Credit: Dr. Durfee

Initial shape of waveHow much energy is contained in each frequency component ( = vk)

Note: frequencies are infinitely close together

Propagation Of Light PulsePropagation Of Light Pulse

E(x,t) Power Spectrum

Credit: Dr. Durfee

Wave moving in time How much energy is contained in each frequency component

Tracking a Moving PulseTracking a Moving Pulse

E(x+vt,t) Power Spectrum

Credit: Dr. Durfee

Graph “window” is moving along with speed v

How much energy is contained in each frequency component

Laser Pulse in Laser Pulse in DispersiveDispersive Medium Medium

Et=0 = sin(10 x)*exp(-x^2) Power Spectrum

Credit: Dr. Durfee


Initial shape of wave

Not all frequency components travel at same speed

Time Evolution of Time Evolution of DispersiveDispersive Pulse PulseCredit: Dr. Durfee

Wave moving in time

Peak moves at about 13 m/s (on my office computer)


Tracking a Tracking a DispersiveDispersive Pulse Pulse

E(x+vgt,t)

Credit: Dr. Durfee

Graph window moving along with peak, ~13 m/s

announcements 10/8/10 prayer exam: last day = tomorrow! a. a.correction to syllabus: on saturdays,...

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