PHY138 – Waves, Lecture 4
Today’s overview
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Sound and Light
Power and Intensity
The Doppler Effect
Medical Applications of Ultrasound
Reading Assignment
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Next week’s reading is Knight Chapter
21, Sections 21.1 – 21.8. There is a preclass 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
5:00PM.
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!
Reflection of Transverse Wave Pulse
(Chapter 21 concept.)
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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 small source, the
intensity is I=P/(4πr2)
Doppler Effect