WAGGA WAGGA CAMPUS
SCHOOL OF CLINICAL SCIENCES
PHY114 MEDICAL RADIATION PHYSICS
TOPIC 1: WAVES, SOUND and OPTICS
EXAMINATION, SPRING 2005
LECTURER: Dr Hans Swan
DAY and DATE: Thursday, 1 September, 2005
TIME: 6.00pm – 9.00pm
WRITING TIME: Three [3] hours
MATERIALS SUPPLIED BY UNIVERSITY:
1 x 12-page answer booklet
MATERIALS PERMITTED IN EXAMINATION:
Battery operated calculator (without print
facility)
NUMBER OF QUESTIONS: Eight (8)
INSTRUCTIONS TO CANDIDATES:
1. Enter your name and student number, and sign in the space provided at the bottom of this
page.
2. This is a closed book examination; therefore no written notes or reference material will be
permitted in the examination room.
3. A formula sheet is provided in the question paper.
4. Answer 4 questions as follows: 2 questions (only) from Part A and 2 questions (only) from
Part B.
5. ALL examination papers are to be returned with ALL examination booklets.
This examination is worth 26% of the total subject assessment.
INSTRUCTIONS TO INVIGILATORS:
1. QUESTION PAPER MAY NOT BE RETAINED BY THE CANDIDATE.
STUDENT NAME: ....................................... STUDENT NUMBER: ............................
STUDENT SIGNATURE: ...............................................................................................
PHY114 Exam: (Topic 1) Spring 05
Page 1 of 5
FORMULA SHEET
(It should be noted that not every equation studied / presented in the Waves, Sound and Optics Topic can be
provided here. Below are some important equations used in the Topic which may be of help to you).
p m   0 c v m
 I 
dB  10 log 10  
 I0 
vm   sm
 c  v0 

f '  f 
 c  vs 
k
sin  1
n
c
 2  1
sin  2
n1
c2
2

c
v
  2 f
n
c f 
1
1
1


p
q
f
B
m   d sin 
c
0

p 

B   

V
V
0 

m  
F kx
m   a sin 
T  2
c
m
k
FT

PHY114 Exam: (Topic 1) Spring 05
q
p
E
1
k A2
2
I
P
A
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PART A: Answer TWO (2) questions (only) from this section.
1. (a) Explain Hooke’s Law and its connection with the phenomenon of Simple
Harmonic Motion.
(8)
(b) A spring stretches 3.9cm when a 10g mass is hung from it.
(i) If a total mass of 25g attached to this spring oscillates in simple harmonic
motion, calculate the period and frequency of motion.
(6)
(ii) If the amplitude of oscillation is 10cm, calculate the total energy of the system
and the maximum speed of the oscillating mass.
(6)
2. (a) A harmonic wave is travelling along a rope. It is observed that the oscillator that
generates the wave completes 40 vibrations in 30.0 sec. Also a given maximum
travels 425cm along the rope in 10.0 sec. What is the wavelength?
(10)
(b) The mass of 1.00m of a certain string is 0.75g. Suppose an 80cm length of this
string is hung vertically with a 400g mass attached to its lower end.
(i) At what speed would waves travel along the hanging string?
(6)
(ii) To what frequency would the string resonate in two segments?
(4)
3. (a) The pressure variation in a sound wave is given by
p 
2.2 cos (
x
3
 1700 t )
where Δp is in Pascals, x is in metres, and t is in seconds. Determine
(i)
(ii)
(iii)
(iv)
the wavelength,
the frequency,
the wavespeed,
the displacement amplitude of the wave.
(3)
(3)
(3)
(5)
Assume the density of the medium is ρ = 2.7 x 103 kg m-3.
(b) Explain the physical properties of a medium which control the speed of sound in it.
Hence explain why the speed of sound in water increases with temperature.
PHY114 Exam: (Topic 1) Spring 05
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(6)
4. (a) A small loudspeaker delivers 1mW of acoustic power. If the power is uniformly
radiated in all directions, what is the sound intensity level at a distance of 5m from
the speaker? Take the threshold of hearing to be I0 = 1.0 x 10-12 Wm-2.
(6)
(b) What would be the intensity level at a distance of 5m if two adjacent speakers were
radiating at the same time, and each was delivering 1mW of power? Show full
working.
(8)
(c) An ultrasound pulse travels through a specific human soft-tissue organ, and suffers
a loss in intensity of -60 dB due to attenuation. If the emitted intensity was
20mWcm-2, what is the final intensity?
(6)
--------------------------------------------------------------------------------------------------------------
PART B: Answer TWO (2) questions (only) from this section.
5. (a) Briefly describe two technological applications of the Doppler Effect.
(6)
(b) A bat is flying at 5.0ms-1 towards a wall and emits a chirp at 40kHz. What
frequency is heard by another bat sitting on the wall (in the direct flight path of the
moving bat)? Assume the speed of sound is 340ms-1.
(8)
(c) What is the frequency of the echo chirp (reflected off the wall) heard by the flying
bat in (b)?
(6)
6. (a) A tube closed at one end resonates to a fundamental sound frequency of 680Hz. The
closed end is now opened. What are the two lowest resonant frequencies for the
newly opened tube? Take the speed of sound in air to be 340ms-1.
(12)
(b) A tuning fork of frequency 256Hz is used to tune a piano-string which is vibrating
in its fundamental mode. As the string is progressively loosened, the beat frequency
increases to 5Hz. What is the frequency of the piano string at this stage? Explain.
(8)
PHY114 Exam: (Topic 1) Spring 05
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7. (a) A submarine is 300m horizontally out from the shore and 100m beneath the surface
of the water. A laser beam is sent from the sub so it strikes the surface of the water
at a point 210m from the shore. If, after refraction at the water surface, the laser
beam just strikes the top of a building standing directly at the water’s edge, find the
height of the building. Take nwater = 1.33 and nair = 1.00.
(8)
(b) An optical fibre consists of a central strand of material surrounded by an outer
coating. The interior portion of the fibre has an index of refraction of 1.60. If all the
rays striking the interior walls of the fibre with incident angles greater than 59.5°
get totally internally reflected, what is the index of refraction of the coating?
(6)
(c) Explain the meaning and purpose of the following camera adjustments. Also
explain their relationship for constant image brightness.
(i)
(ii)
shutter speed
f- number
(6)
8. (a) A converging lens has a focal length of 20.0cm. An object 2.00cm tall is placed
30cm in front of the lens. Using the lens equation, locate and fully describe the
image (i.e. real/virtual, upright/inverted, magnification), giving reasons for your
answers.
(8)
(b) White light is spread out into its spectral components (colours) by a diffraction
grating. On a screen 1.5m beyond the grating, a particular red colour appears in first
order, 19.35cm from the straight-through position. If the grating has 2000 lines per
centimetre, what is the wavelength of the light in nanometres?
(6)
(c) Explain why you can hear, but not directly see around corners.
(6)
PHY114 Exam: (Topic 1) Spring 05
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