P a g e | 77
Author: Mr Endunde
4. Properties of waves
4.5 General wave properties
Waves
A wave may be thought of as a spreading of disturbance from one place to another.
The source of any wave is oscillation or simple harmonic motion
A wave motion (propagation), is defined as an oscillation which transfers energy from one place to
another without any net movement of the medium.
An oscillation or simple harmonic motion is defined as any repeated to- and fro- motion of a fluid or
elastic solid, e.g. turning fork, pendulum or stretching string.
Examples of wave motion
(a) Pulses (i.e. short bursts or disturbances) of ripples are obtained by dipping a finger in the water for
circular ripples.
(b) Moving a rope up and down while fixing one end to the pole.
(c) Pulling and pushing the slinky spring attached to a fixed point at one end.
PHYSCS GRADE 10 TO 11 MARSS 2019 Mr Endunde
P a g e | 78
Author: Mr Endunde
Types of waves
All waves can be classified into two groups:
(a) Longitudinal wave
(b) Transverse waves
Describing waves
Terms used to describe waves can be explained with the aid of a displacement–distance graph
(a) Crest and trough
These are the high points and low points that characterize transverse waves only. For longitudinal
waves, the terms ‘compressions’ and ‘rarefactions’ are used.
(b) Wavelength
The wavelength of a wave, represented by the Greek letter λ (‘lambda’), is the distance between
successive crests.
If there are 10 waves in 5 meters then the wavelength is 0.5m
(c) Frequency
The frequency f is the number of complete waves generated per second.
If there are 40 waves in 10 seconds then the frequency is 4Hz
PHYSCS GRADE 10 TO 11 MARSS 2019 Mr Endunde
P a g e | 79
Author: Mr Endunde
(d) Period
Period, T is time taken to produce one complete wave.
If there are 40 waves in 10 seconds then the period is 0.25 s
1
𝐓=
𝑓
(e) Speed
Speed v, is the distance travelled by the wave in 1 second.
If the wave has a wavelength of 0.5 m and frequency of 4 Hz then speed is 2 m/s
V= f λ
(f) Amplitude
The amplitude a is the height of a crest or the depth of a trough measured from the undisturbed
position of what is carrying the wave.
(g) Phase
The short arrows at A, B, C, D on the figure above shows the directions of vibration of what is
carrying the wave, such as a rope. The parts at A and C have the same speed in the same direction
and are in phase. At B and D the parts are also in phase with each other but they are out of phase
with those at A and C because their directions of vibration are opposite.
(h) Wave front
In two dimensions, a wave front is a line containing adjacent point that are in the same phase.
Worked example
The speed, c of green light of wavelength 0.9 µm in vacuum is 3.0 × 108 m/s. Calculate the frequency
Solution
Given:
Wavelength = 0.9 µm = 0.9 × 10-6 m
Speed, c = 3.0 × 108 m/s
c
𝑓=
λ
3.0 × 108
𝑓=
0.9 × 10−6
𝑓 = 5.0 × 1014 Hz
PHYSCS GRADE 10 TO 11 MARSS 2019 Mr Endunde
P a g e | 80
Author: Mr Endunde
Wave production and the ripple tank
In a ripple tank, plane waves can be set by using a straight dipper made up of either wood or plastic
while circular waves can be formed by using a spherical dipper made of plastic.
Plane waves and circular waves are produced by using a motor to cause the vibrations or
oscillations.
These waves will be seen as bright and dark on the screen below the tray. The dark lines show the
position of the crest.
Reflection of water waves in the ripple tank
The figure below shows straight water waves are falling on a metal strip placed in a ripple tank at an
angle of 60°, i.e. the angle i between the direction of travel of the waves and the normal to the strip
is 60°, as is the angle between the wavefront and the strip. (The perpendicular to the strip at the
point where the incident ray strikes is called the normal.)
The wavefronts are represented by straight lines and can be thought of as the crests of the waves.
They are at right angles to the direction of travel, i.e. to the rays. The angle of reflection r is 60°.
Incidence at other angles shows that the angle of incidence and angle of reflection are always
equal.
Speed, wavelength and frequency are unchanged by reflection
PHYSCS GRADE 10 TO 11 MARSS 2019 Mr Endunde
P a g e | 81
Author: Mr Endunde
Refraction
Speed and wave length is reduced but frequency stays the same and the wave changes direction
Waves slow down when they pass from a less to a more dense material and vice versa
When wave is slowed down, it is refracted towards normal (i > r)
When wave is speed up, it is refracted away from normal (i < r)
Deep water is less optical dense than shallow water
When water wave travels from deep to shallow; speed decreases, wavelength decreases and
frequency remains constant
When water wave travels from shallow to deep; speed increases, wavelength increases and
frequency remains constant
4.6 Light
Luminous and non-luminous objects
Any object (such as the sun, electric lamps or lighted candle) that makes its own light is called
luminous object.
Conversely, any object (such as a table, wall or picture) that do not make its own light but reflect
light is called non-luminous object.
PHYSCS GRADE 10 TO 11 MARSS 2019 Mr Endunde
