9/22/20 Tan TY
1
General Wave Properties
a)
Describe what is meant by wave motion as illustrated by
vibration in ropes, springs and by waves a ripple tank.
b)
Show understanding that waves transfer energy without
transferring matter.
c)
Define speed, frequency, wavelength period and
amplitude.
d)
Show understanding of the term wavefront.
e)
Recall and apply the relationship velocity = frequency x
wavelength to new situations or to solve related problems
f)
Compare transverse and longitudinal waves and give
2
Waves
A wave is a phenomenon in which energy is transferred
through vibrations.
– It is the spreading of a disturbance from one place to another. – The source of any wave is a vibration or oscillation.
Rope Spring
Ripple tank
Examples includes:
– Sound waves from hi-fi – Ripples in a pond
3
Ripple Tank
A ripple tank is a very useful setup to study the behaviour of waves. It
consist of a light source and a transparent tray filled with water.
A horizontal bar (or circular points) dipped in water will produce
transverse waves. A ball-ended dipper will produce circular waves.
When the light source is above the tray the following can be observed:
– A series of light fringes followed by dark fringes. These are the crest
and troughs
Video 1
4
Waves
Waves can generally be classified in two broad
categories:
–
Transverse waves.
Waves which travel in a direction perpendicular to the
direction of travel of the vibrations of the particles.
Crest and trough
–
Longitudinal waves.
Longitudinal waves travel in a direction parallel to the
direction of the vibrations of the particles.
Compressions and rarefaction
animation
5
Wave Motion
Be able to predict for both waves:
– Particle position – Wave shape
6
Waves
Terms to be familiar with:
Terms to be familiar with
Wavefront An imaginary line that joins all the peaks of a wave or all identical points on a wave that are in phase
Crest (peak) The highest points of a wave.
Amplitude The maximum displacement from rest position of a particle
Wavelength ()
- The distance between two successive crests or troughs.
7
Waves
Terms to be familiar with
Frequency (f)
- The number of crest that pass a point per second.
- It is also the number of complete waves generated per second.
Period (T)
- The time taken to generate one complete wave.
-Time of any point of the wave to move a distance of one wavelength.
Speed
8
Displacement/ cm
Time/ s Displacement/ cm
Distance/ cm Displacement – distance graph
amplitude wavelength
Displacement – time graph
amplitude period
9
Waves
Looking a wave at one particular instant, the following can
be observed:
– Each of the particle is oscillating about a mean position, the rest
position.
– In a wave, energy is transmitted, there is no transfer of particles
along with the wave.
Formula:
– T = 1 / f – v = f
10
Waves
A vibration of 5.0Hz sends a wave of wavelength 0.8m
down a rope. What is the speed of the wave?
Ans: v = f
= 5 x 0.8 = 4.0 ms-1
A wave of frequency 300Hz emits a wave of wavelength
0.5m. How long does the wave take to travel 600m
Ans: v = f t = distance / velocity = 300 x 0.5 = 600 / 150
= 150 ms-1 = 4s
More examples on pg. 272
11
Waves
Look at example on pg. 270
12
Practice
1. Waves can be broadly classified into two categories, longitudinal and transverse waves.
a. Distinguish between a longitudinal wave and a transverse wave. [2]
b. Define the following: i. Wavefront
ii. Amplitude
iii. Frequency [3]
c. Draw a full-scale diagram of a transverse wave with an amplitude of 2.0cm and wavelength of 6.0 cm. Your diagram should represent the waveform at a
particular instant and include at least two complete waves. Label this wave as
wave a. [3]
d. Sketch on the displacement time graph, a wave with an amplitude a frequency f, label this wave as wave a. Draw another wave with the same amplitude,
13
Ripple Tanks
Reflection
The normal is an imaginary linethat is perpendicular to the plane surface.
The angle of incidence is
measured from the normal to the direction of travel of the incident wave. Similarly, the angle of reflection is the angle between the normal and the direction of travel of the reflected wave.
Thus the angle of incidence is
equal to the angle of reflection.
If the wavefront are incident
14
Ripple Tanks
Refraction
From deep to shallow:
– wavelength becomes
shorter
– decrease in speed of
the wave
– waves bends towards
the normal
– frequency remains
the same
From shallow to deep:
– wavelength becomes
longer
– increase in speed – waves bends away
from the normal
– frequency remains
the same
15
Circular waves
Perpendicular distance from S to barrier = to perpendicular
distance from S’ to barrier
Reflected wave appears flatter
16
Wave Diagram
A wave diagram is used to represent the characteristic of the
wave accurately.
When drawing such a diagram, ensure the following:
– The direction of travel of the wave is always perpendicular to
the wavefront.
– The normal is clearly marked.
– When waves travel at constant speed, the wavefront should
be equally spaced and parallel to each other.
– Mark clearly the normal, the angles, the direction of travel
17
Challenge
Plane waves are being produced as shown in the diagram
below. Sketch the wavefront when the wave moves from
the shallow to the deep region.
[5]
i.
18
Challenge
1. The figure shown below is drawn to scale. Water waves are being refracted as they cross the boundary. The parallel lines represent the wavefronts. The incident waves have a wavelength of 0.7cm and a frequency of 15Hz.
a. Determine the following:
i. The speed of the incident wave. ii. The period of the refracted wave.
iii. The wavelength of the refracted wave.
iv. The speed of the refracted wave. [4]