UNIVERSITY OF MALAWI
PHYSICS DEPARTMENT
DATE: MAY 2023 TIME ALLOWED: 3 hours
PHY 121: VIBRATIONS AND WAVES, AND ELECTRICITY &
MAGNETISM
MAIN END OF SEMESTER EXAMINATION
Instructions
1. This examination question paper contains ten (10) printed pages, a total of five (5)
questions.
2. Answer all two questions from Section A and all three (3) questions from Section
B.
3. The relative allocation of each part-question is indicated in the right hand margin
You may not start to read the questions printed on the subsequent pages on this
question paper until instructed to do so by the invigilator.
[PLEASE TURN OVER]
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LIST OF PHYSICAL CONSTANTS
Pressure of a standard atmosphere 1013 105 Pa (or N m−2 )
Speed of light (in vacuum) c 3 108 m s−1
Speed of sound in air va 340 m/s
Speed of sound in water vw 1480 m/s
[PLEASE TURN OVER]
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Section A
VIBRATION AND WAVES
(Answer ALL questions from this section)
QUESTION 1: Compulsory
Write only the letter representing the correct answer for each question. Do not rewrite the
question. Each question is worth 1 mark.
1.1 . A Standing wave experiment is performed to determine the speed of waves in a
rope. The standing wave pattern shown below is established in the rope. The rope
makes exactly 90 complete vibrational cycles in one minute. The speed of the
waves is ____ m/s.
Fig. 1
A. 3.0
B. 6.0
C. 180
D. 360
E. 540
1.2 . The best definition of refraction of light is ____.
A. Passing through a boundary B. Bouncing off a boundary
C. Changing speed at a boundary D. Changing direction when crossing a boundary
1. 3 If carbon tetrachloride has an index of refraction of 1.461, what is the speed of light
through this liquid?
A. 4.38 x 108 m/s B. 2.05 x 108 m/s
C. 4.461 x 108 m/s D. 1.461 x 108 m/s
1.4 The diagram in Fig. 3 shows a disturbance moving through a rope towards the
right. If this disturbance meets a similar disturbance moving to the left, then
which one of the diagrams below depict a pattern which could NEVER appear in
the rope? Answer is D
Fig. 2
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1.5 A transverse wave is travelling through a medium as shown in Fig. 4. The particles
of the medium are vibrating _____.
Fig. 3
A. parallel to the line joining AD.
B. along the line joining CI.
C. perpendicular to the line joining AD.
D. at various angles to the line CI.
E. along the curve CAEJGBI.
1.6 As a wave travels into a medium in which its speed increases, its wavelength would
____.
A. decrease
B. increase
C. remain the same
1.7 Which of the following is a common characteristic of visible light, ultraviolet
rays, infrared rays, gamma rays and X-rays?
A. Influenced by magnetic field.
B. Possess the same frequency.
C. Possess the same wavelength.
D. Possess the same velocity in vacuum.
1.8 Which of the following statements about longitudinal wave is correct?
A. Particles vibrate at right angles to the movement of the wave.
B. Particles move in the same direction to the propagation of the wave.
C. Particles sometimes move in the opposite direction to the propagation of the wave.
D. No transmission medium is required.
1.9 What is the amplitude of the wave in the diagram shown in Fig. 5?
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Fig. 4
A. 0.03 m. B. 0.04 m. C. 0.05 m. D. 0.06 m.
1.10 The diagram in Fig. 6 shows a circular water wave travelling towards a straight
barrier in a ripple tank. Which of the following diagrams shows the correct
reflected wave? Answer is A
Fig. 5
1.11 If two waves are of the same frequencies, which of the following characteristics is
identical for the waves?
A. Phase
B. Wavelength
C. Amplitude
D. Period
1.12 Which of the following diagrams (shown in Fig. 7) is correct when the water waves
travel from a deep region to a shallow region in a ripple tank? Answer is D
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Fig. 6
1.13 A hand-held stroboscope is used to freeze a water wave of speed 5 cm s−1. There are 3
slits on the stroboscope and the wave appears to be stationary when the stroboscope
rotates at a speed of 0.5, 1, 1.5 and 3 revolutions per second only. What is the frequency
of the wave?
A. 1.5 Hz
B. 3 Hz
C. 9 Hz
D. 15 Hz
1.14 Two waves are produced by a vibrator. Wave A has an amplitude larger than wave B .
The energy transported by wave A is __________ that transported by wave B.
A. larger than
B. the same as
C. smaller than
D. It cannot be determined
1.15 The Fig. 8 shows two water waves which are generated by two coherent sources S1 and
S2. The solid lines represent wave crests and the dashed lines represent wave troughs.
Which point is at a position of destructive interference?
Fig. 7
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A. B
B. C
C. A
D. D
1.16 Which of the following settings in a ripple tank will produce the most prominent
diffraction pattern of a water wave of wavelength of 2 cm?
A. The wave passes through an obstacle of length of 4 cm.
B. The wave passes through an obstacle of length of 2 cm.
C. The wave passes through a gap of 4 cm.
D. The wave passes through a gap of 1 cm.
1.17 If the critical angle for internal reflection inside a certain transparent material is
found to be 48.0 degrees, what is the index of refraction of the material? (Air is
outside the material).
A. 1.35 B. 1.48 C. 1.49 D. 0.743
1.18 An object is placed at a distance of 30.0 cm from a thin converging lens along its axis.
The lens has a focal length of 10.0 cm. What are the values of the image distance and
magnification (respectively)?
A. 60.0 cm and 2.00 B. 15.0 cm and 2.00
C. 60.0 cm and 0.500 D. 15.0 cm and 0.500
1.9 In which of the following waves will interference occur?
(1) Sound (2) Light (3) Water
A. (1) only
B. (2) only
C. (1) and (3) only
D. (1), (2) and (3)
1.20 A single disturbance that moves from point to point through a medium is called a ___.
A. Period
B. Periodic wave
C. Wavelength
D. Pulse
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QUESTION 2
(a) A Two very small, identical loudspeakers, each of which is radiating sound
uniformly in all directions, are placed at points S1 and S2 as shown in the
diagram in Fig. 8. They are connected to a source so that they radiate, in phase, a
sound with a wavelength of 2.00 m. The speed of sound may be taken as 340
m/s.
(i) What is the frequency of the radiated sound? [2 marks]
(ii) Point M, a nodal point, is 7.0 m from S1 and more than 7.0 m from S2. What
are three possible distances M could be from S2? [4 marks]
(iii) Point N, also a nodal point, is located 12.0 m from S1 and 5.0 m from S2.
On what nodal line is it located? [2 marks]
Fig. 8
Solution
(i) S
(ii) Ans
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(iii) Ans
(b) The Fig. 10 shows a wave front pattern in a ripple tank produced by a vibrating
dipper at frequency of 5 Hz. What is the wave speed? [4 marks]
Fig. 10
Solution:
(c) A Convex mirror has a 20 cm radius of curvature. Find the image distance for
the mirror when the object distance is 6.33 cm. [4 marks]
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(d) Carbon disulfide (n = 1.63) is poured into a container made of crown glass (n =
1.52). What is the critical angle for internal reflection of a ray in the liquid when
it is incident on the liquid-to-glass surface? [4 marks]
Solution
The critical angle is the angle of incidence (which is always in the more dense material) for
which the angle of refraction is 90 degrees.
Apply this to Snell's law equation:
1.63* sine(critical) = 1.52 * sine (90)
Solve for Theta critical.
sine((critical) = 1.52/1.63 = 0.9325
critical. = invsine(0.9325) = 68.8 degrees
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Section B:
Electricity & Magnetism
(Answer ALL three (3) questions from this section)
3 (a)
END OF EXAMINATION PAPER
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