(12) General Wave Properties

Question 1

(D) Define a wave/wave motion.

Answer 1

1. Waves are defined as a periodic motion /travelling disturbance
2. Repeated at regular intervals
3. Which transfers energy from one point to another
4. Without any transfer of matter or medium

Question 2

(D) Define an oscillation.

Answer 2

An oscillation is defined as a complete to-and-fro motion

From one extreme position to the other extreme position, and back

Question 3

(D) Define a crest.

Answer 3

A crest is defined as the highest point of a transverse wave.

Question 4

(D) Define a trough.

Answer 4

A trough is defined as the lowest point of a transverse wave.

Question 5

(D) Define a Wavefront.

Answer 5

A wavefront is defined as an imaginary line joining all points on the waves that are in phase.

Question 6

(Q) The direction of travel of the waves is ________ to the wavefront.

A. Parallel
B. Perpendicular

(treat it as 2D, from the top view)

Answer 6

(Ans: B)

The direction of travel of the waves is PERPENDICULAR to the wavefront.

Question 7

(Q) When are any 2 points of a wave in phase? (2 factors)

Answer 7

Any 2 points of a wave are in phase when they:
1. Move in the same direction
2. Have the same speed and displacement from their rest position
(Also note: when they are exactly 1 period apart -eg like crest and crest)

Question 8

(D) Define transverse waves.

Answer 8

Waves where the direction of vibration of particles is is perpendicular to the direction of propagation of the wave.

Question 9

(D) Define longitudinal waves.

Answer 9

Waves where the direction of vibration of particles is is parallel to the direction of propagation of the wave.

Question 10

(Q) Give examples of transverse waves (Name 3)

Answer 10

Any 3 from
Any of the EM waves
1. Radio waves
2. Microwaves
3. Infra-red
4. Visible light
5. Ultraviolet
6. X-rays
7. Gamma rays

Question 11

(Q) Give an example of a longitudinal wave.

Answer 11

Sound wave

Question 12

(Q) For longitudinal waves, what is a compression and rarefraction?

Answer 12

Compression: place where particles are the closest to one another
- Places in longitudinal wave where air pressure is slightly higher than the surrounding air pressure (P/V relationship: Pressure is inversely proportionate to volume, so since particles squashed together –> lower volume –> higher air pressure)

Rarefraction: place where particles are spread the furthest apart
- Places in longitudinal wave where air pressure is slightly lower than the surrounding air pressure. (P/V relationship: particles far apart –> high volume –> lower air pressure)

Question 13

(Q) What is one wavelength for a longitudinal wave?

Answer 13

From compression to compression OR
from rarefraction to rarefraction

(As centre of Compression and centre of Rarefraction do not change as “spring” is stretched)

Question 14

(D) Define Wavelength. (入)

Answer 14

A wavelength is defined as the shortest distance between any two points in phase in a wave.

Question 15

(D) Define amplitude.

Answer 15

Amplitude is defined as the maximum displacement (of a particle) from the rest position in a wave.

Question 16

(D) Define Period. (T)

Answer 16

Period is defined as the time taken for one complete oscillation/wave.

Question 17

(D) Define frequency. (f)

Answer 17

Frequency is defined as the number of complete oscillations per unit time.

f = 1/T (1/Period)

Question 18

(D) State the General wave equation.

Answer 18

v = f入

v: velocity of the wave
f: frequency
入: wavelength

Question 19

(Q) (Carrying out a wave expt in water tank.) If there is a piece of thick plastic in the middle of the water tank, when the waves come to the middle, they
___________ and ______________

A. Slow down and wavelength decreases
B. Slow down and wavelength increases
C. Speed up and wavelength decreases
D. Speed up and wavelength increases

Answer 19

Ans: A

When the waves come to the part where below is plastic, they slow down and wavelength decreases.

But what comes first?
Ripples SLOW DOWN in shallower water (with plastic) –> as a result wavelength decreases

However, I cannot tell that v decreases using my eye. But I observe that 入 decreases. So I can deduce that a decrease in v happened in order to cause the decrease in 入.

DOES THE FREQUENCY CHANGE?
No. As it is determined by the dipper.

Question 20

(Q) Setup 1:
Dipper at f = 1 Hz
Water tank with 30cm depth water

Setup 2:
Dipper at f = 2Hz
Water tank with 30cm depth water.

The wavelength of setup 1 is _____than setup 2.

A: Higher
B: Lower

Answer 20

Ans: A

If Setup 1:
v = f入

Then Setup 2:
v = f(higher) x 入(lower)

v does not change as it is dependent on the medium. The same medium has been used. So for the same v, and a higher frequency, setup 2 needs to have a SHORTER wavelength than setup 1 to compensate for the increase in frequency.

Question 21

(Q) Drawing wave diagrams:
If incident waves going towards shallow water, what direction does the wave bend to? (refract)

A: Towards the normal
B: Away from the normal

Answer 21

Ans: A

Waves bend toward normal in shallower water.
(Note: remember to draw wavefront – which is perpendicular to direction of wave. Wavefronts must be equidistant)

Question 22

(Q) What can you find from a displacement-distance graph?

Answer 22

1. WAVELENGTH, 入

2. Amplitude (reading off graph)

Question 23

(Q) What can you find from a displacement-time graph?

Answer 23

1. PERIOD, T
2. Hence the frequency can be found as 1/T
3. Amplitude (reading off graph)

Question 24

(Q) How can particles have the same disp/dist graph and/or disp/time graph?

Answer 24

Particles must be in phase.

Question 25

(Q) Which of the following statements about longitudinal waves is correct?

A. Particles vibrate at right angles to the movement of the waves.
B. The particles move in the same direction to the propagation of the waves.
C. The particles sometimes move in the opposite direction to the propagation of the waves.
D. No transmission medium is required.

Answer 25

ANS: C

A. Particles vibrate at right angles to the movement of the waves.
This only applies for transverse waves. Longitudinal waves: parallel

B. The particles move in the same direction to the propagation of the waves.
Yes, but it can also move in opp direction: like a spring recoiling.
‘(in physics, the centre of compression and centre of rarefraction remains where it is, but since in the COC particles are vv close, next instant it will spread out again. Likewise for COR, particles vv far apart, next instant will squash up. So they go in both directions back to the COR/ away from COC)

C. The particles sometimes move in the opposite direction to the propagation of the waves.
Correct ans. expln above.

D. No transmission medium is required.
only for EM waves: can in vacuum (and EM wave is a kind of transverse wave)