PHY138 – Waves, Lecture 4
The Plan for Today:
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Sound and Light
Medical Applications of Ultrasound
Power and Intensity
The Doppler Effect
Meet & Greet
Friday, Nov. 16th @ 4:00pm
MSB 3153
Talk to medical students from UofT to learn about
their experiences in applying to medical school.
Food and drinks will be served.
A free Kaplan MCAT course will be raffled off!
*****$5 membership fee will be charged for non-members. This will provide access to all of the
upcoming events for 2007-2008.*****
A clipboard is being passed around so that students can add their email
address to the Pre-med society email list. They will receive emails from the
society regarding any upcoming events and important announcements.
Reading Assignment
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Next week’s reading is Knight Chapter 21,
Sections 21.1 – 21.8. There is a pre-class
quiz on www.masteringphysics.com for this
material due on Monday morning.
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A www.masteringphysics.com Problem Set on
Chapter 20 is due on Friday at 11:59PM.
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A Written Team Problem Set is due next
Friday, Nov.24 at 5:00 PM. It’s available in
PDF Format on the Waves Class Summaries
Page.
Sound and Light
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Sound is a pressure wave in a gas, liquid or
solid. Speed depends on material.
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Light is one type of electromagnetic wave.
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In a vacuum, all electromagnetic waves
(including light) travel at c = 3×108 m/s.
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In transparent media, light slows down. Index
of Refraction is n > 1.
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This reduces the wavelength, but does not
change the frequency!
A light wave travels through three transparent
materials of equal thickness. Rank is order, from
the largest to smallest, the indices of refraction
n1, n2, and n3.
A. n2 > n1 > n3
D. n3 > n2 > n1
B. n3 > n1 > n2
E. n1 = n2 = n3
C. n1 > n2 > n3
Reflection of Transverse Wave Pulse
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A pulse traveling to
the right on a heavy
string attached to a
lighter string
Speed suddenly
increases
Reflection of Transverse Wave Pulse
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A pulse traveling to
the right on a light
string attached to a
heavier string
Speed suddenly
decreases
Physics of Ultrasound
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Speed of sound in bone, flesh and blood
are all different
When the speed of any wave suddenly
changes, there is a reflection and
transmission
Ultrasound images are formed from
reflected high frequency sound
Image resolution is set by wavelength, λ
λ=v/f, so higher frequency yields smaller λ,
and better resolution
Speed of sound in humans
Tissue
Sound Speed (m/s)
Air
350
Fat
1450
Brain
1540
Blood
1570
Bone
4080
Muscle
1585
Power and Intensity
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The Power, P, of any wave source is how
much energy per second is radiated as
waves [units = Watts]
The Intensity, I, is the energy rate per area.
This determines how loud (sound) or bright
(light) the wave is.
I=P/a, where a is an area perpendicular to the
wave direction.
At a distance r from a spherically symmetric
source, the intensity is I=P/(4πr2)
Chapter 20, Problem 34
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The sound intensity from a jack hammer
breaking concrete is 2 W/m2 at a distance
of 2 m from the point of impact. This is
sufficiently loud to cause permanent
hearing damage if the operator doesn’t
wear ear protection. What is the sound
intensity for a person watching from 50 m
away?
Doppler Effect
Which statement is true?
Valerie is standing in the middle of the road, as
a police car approaches her at a constant
speed, v. The siren on the police car emits a
“rest frequency” of f0.
A. The frequency she hears rises steadily
as the police car gets closer and closer.
B. The frequency she hears steadily
decreases as the police car gets closer
and closer.
C. The frequency she hears does not
change as the police car gets closer.
Which statement is true?
Valerie is standing still as a police car approaches
her at a constant speed, v. Daniel is in his car
moving at the same constant speed, v, toward an
identical police car which is standing still. Both
Actual Answer!
hear a siren.
Check eq.20.38
A. The frequency Daniel hears is lower and 20.39!
than the frequency Valerie hears.
B. The frequency Daniel hears is
higher than the frequency Valerie
hears.
(Almost correct)
C. The frequencies that Daniel and
Valerie hear are exactly the same.