Light and Sound
Lecture 35
Reading: 15: 4-6
Problems: 15: 1, 4, 7, 13, 15, 17, 18
Quiz
• vSOUND = 343 m/s
• The little bat in the ultrasonic ranger sends
out a short 40 khz pulse. It bounces of a
target 1.7m away. How much time elapses
before he hears the reflected echo?
33%
33%
.5
6m
8.
6
m
m
33%
17
1. 8.6 mm
2. 17 m
3. .56m
60
m
What is the wavelength of a 40kHz
sound wave?
Ultrasound
• Vwater = 1480 m/s
• Objects can be imaged to an approximate
level of detail of about one wavelength. What
frequency is needed to obtain 0.5 mm
resolution of an object in a human body?
Ultrasound
• Solution
f v
4
  0.5 m m  5. *10 m
f 
v


1480 m / s
5 *10
4
m
 2.96 *10 H z  3 M H z
6
HeNe Laser
• The wavelength of light from a Helium-Neon
laser is 6328 angstroms. 1 angstrom = 10-10 m
• What is the frequency of this light?
Intensity
• Intensity
I 
P W atts
a
m
2
• Point source (that is, we are at a distance large
compared to the source dimension)
I (r ) 
PSO U R C E
4 r
2
Sound
• Increasing sound intensity by a factor of 10 is
perceived as an increase in loudness by a
factor of 2. (Ask your biology professor to
explain this.)
• Decibel scale to measure sound intensity level
• Reference is lowest intensity you can perceive
in an extremely quiet room
I 0  1.0 *10
 12
W
m
2
Decibel Scale
• Define
  (10 d B ) log 10
 I 


I
 0 
• Measure loudness relative to threshold of
hearing
Problem 15:25
• Sound is detected when a sound wave causes
the eardrum to vibrate. Typically the diameter
of the eardrum is about 8.4mm. How much
energy is delivered to your eardrum each
second when someone whispers (20dB) a
secret in your ear?
15:25
  (10 d B ) log 10
 I 


 I0 
I
20  (10 d B ) * log
I0
 I 
log 
2
 I0 
10
 I 
log 

 I0 

I
 10  100
2
I0
I  100 I 0  100 *10
 12
W
m
2
 10
 10
W
m
2
15:25
• Power is W = J/s=I*a
P  Ia  10
10
W
m
5.5 *10
 15
W
*  (.0042 m ) 
2
2
15:27
• The intensity of electromagnetic waves from
the sun is 1.4kW/m2 just above the earth’s
atmosphere. Eighty percent of this reaches
the earth’s surface on a clear day. Model your
back as 30cm x 50 cm rectangle. How many
joules of solar energy fall on your back as you
work on your tan for 1.0 hr?
15:27
• Intensity at earth’s surface
I S  0.80 *1.4
kW
m
2
 1.12
kW
m
2
 1120
W
m
2
P  Ia  1120 * (.30 * .50)  168W  168
J
s
E  P (  t )  168
J
s
(3600
s
hr
)  6 *10 J
5