PHY138

1
PHY138 Waves, Lecture 4The Plan for Today

• Sound and Light
• Medical Applications of Ultrasound
• Power and Intensity
• The Doppler Effect

2
Reading Assignment

• Next weeks 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.
• A www.masteringphysics.com Problem Set on Chapter
  20 is due on Friday at 1159PM.
• A Written Team Problem Set is due next Friday,
  Nov.24 at 500 PM. Its available in PDF Format
  on the Waves Class Summaries Page.

3
Sound and Light

• Sound is a pressure wave in a gas, liquid or
  solid. Speed depends on material.
• Light is one type of electromagnetic wave.
• In a vacuum, all electromagnetic waves (including
  light) travel at c 3108 m/s.
• In transparent media, light slows down. Index of
  Refraction is n gt 1.
• This reduces the wavelength, but does not change
  the frequency!

4
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 gt n1 gt n3 B. n3 gt n1 gt n2 C. n1
gt n2 gt n3 D. n3 gt n2 gt n1 E. n1 n2 n3
5
Reflection of Transverse Wave Pulse

• A pulse traveling to the right on a heavy string
  attached to a lighter string
• Speed suddenly increases

6
Reflection of Transverse Wave Pulse

• A pulse traveling to the right on a light string
  attached to a heavier string
• Speed suddenly decreases

7
Physics of Ultrasound

• 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, ?
• ?v/f, so higher frequency yields smaller ?, and
  better resolution

8
Speed of sound in humans
Tissue Sound Speed (m/s)
Air 350
Fat 1450
Brain 1540
Blood 1570
Bone 4080
Muscle 1585
9
Power and Intensity

• 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.
• IP/a, where a is an area perpendicular to the
  wave direction.
• At a distance r from a spherically symmetric
  source, the intensity is IP/(4pr2)

10
Chapter 20, Problem 34

• 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
  doesnt wear ear protection. What is the sound
  intensity for a person watching from 50 m away?

11
Doppler Effect
12
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.

1. The frequency she hears rises steadily as the
   police car gets closer and closer.
2. The frequency she hears steadily decreases as the
   police car gets closer and closer.
3. The frequency she hears does not change as the
   police car gets closer.

13
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 hear a siren.
Actual Answer! Check eq.20.38 and 20.39!

1. The frequency Daniel hears is lower than the
   frequency Valerie hears.
2. The frequency Daniel hears is higher than the
   frequency Valerie hears.
3. The frequencies that Daniel and Valerie hear are
   exactly the same.

(Almost correct)