Chapter
17
Wave
Phenomena
Transverse or longitudinal waves transport energy from one point to
another. Each particle in the medium vibrates or oscillates, and
disturbs the neighbouring particles in a ‘domino-like’ effect.
What happens when two waves meet at a point? How does the
particle of the medium react at that point?
Chapter 17: Wave Phenomena
Ripple Tank
The basic component is a large transparent tray and a bar suspended by
elastic bands and attached to a motor. The motor vibrates the bar to create
waves on the surface of the water. The frequency of the wave is determined
by the motor.
Chapter 17: Wave Phenomena
When a series of waves form on the surface of the water, the crests act as
lenses that focus the light rays to form bright lines on the screen.
Dark lines
Bright lines
Troughs
Crests
The distance between
the dark lines or the
distance between the
bright lines is equal to
the wavelength of the
wave train on the water
surface.
Chapter 17: Wave Phenomena
Plane Wave
A plane wave has parallel wave
fronts that have constant
wavelength and amplitude.
It is made up of similar wave
trains moving in the same
direction.
Direction of wave motion
Crest
A plane wave - wave trains travelling side by side
Circular Wave
A circular wave has circular wave fronts
that expand outward.
It is made up of wave trains moving
outwards from a single point.
Chapter 17: Wave Phenomena
Image of wave pattern
Reflection
A wave can be ‘bounced’ off the surface of
an obstacle or barrier.
The incoming wave is called the incident
wave and the ‘bounced’ off wave is called
the reflected wave.
The angle between the direction
of motion of the incident wave
and the normal is called the
angle of incidence, i.
The angle between the direction
of motion of the reflected wave
and the normal is called the
angle of reflection, r.
angle of incidence = angle of reflection
Chapter 17: Wave Phenomena
The following diagrams show how a circular wave front from a point source (S1)
is reflected off a plane surface.
The reflected wave front seems to come from another point source (S2) behind
the reflected surface.
S1 .
S2 .
S1 .
(a)
(b)
S2 .
S1 .
(c)
S2 .
S1 .
S2 .
(d)
A circular wave front being reflected off a flat surface.
Chapter 17: Wave Phenomena
Refraction
Refraction is caused by the change in the speed of a wave when it
travels from one medium to another.
Water waves slow down when they travel into shallower regions.
The frequency does not change. Only the wavelength is affected by
the change in speed.
Chapter 17: Wave Phenomena
If the block is angled such that neighbouring wave trains do not cross
into the shallower region at the same time, we will observe the wave
bending towards the normal.
The normal is the line perpendicular to the
line separating the regions of different
depths.
The angle between the direction of motion
of the incident wave and the normal is
called the angle of incidence i
The angle between the direction of motion
of the refracted wave and the normal is
called the angle of refraction r.
sin i / sin r = vi / vr = λi / λr
Chapter 17: Wave Phenomena
where
i is the angle of incidence,
r is the angle of refraction,
vi is the speed of the incident wave,
vr is the speed of the refracted wave,
λi is the wavelength of the incident wave and
λr is the wavelength of the refracted wave.
Diffraction
When a wave passes the edge of an obstacle, it spreads out from its original
direction of motion. This phenomenon is called the diffraction of waves.
Wooden bar of a ripple tank
Obstacle
Wave front bending away from original direction
Spreading of water waves around the edge of an obstacle in a ripple tank
The bending of waves is more
pronounced when the size of
the obstacle is comparable to
the size of the wavelength.
Diffraction of waves is more pronounced when the
opening is comparable to the wavelength.
Chapter 17: Wave Phenomena
Chapter 17: Wave Phenomena
Superposition of waves
Superposition of waves is the addition of the amplitudes of two or more waves that
meet at a point.
When waves move through a medium, the particles react by oscillating about their
equilibrium position.
When two or more waves meet at a point, the particles at that point oscillate corresponding
to the resultant of the waves’ amplitudes.
(a) Two pulses of the same amplitude, A, approach
each other along the same medium.
(b) When they meet, they combine to form a
resultant amplitude.
(c) In this instance, they combine to form an
amplitude that is twice the individual amplitudes.
(A + A = 2A).
(d) The pulses pass each other with their original
amplitudes.
Superposition of approaching waves of same amplitude
Chapter 17: Wave Phenomena
Interference
Coherent waves
When waves of similar wavelength, amplitudes and frequency
meet, they form an interference pattern due to the superposition of waves.
The diagram shows waves
produced by coherent sources.
Waves produced by coherent
sources are generated in phase,
and have the same frequency and
wavelength.
Coherent sources produce welldefined interference patterns.
Chapter 17: Wave Phenomena
The black dotted lines are called
anti-nodal lines. The red dotted
lines are the nodal lines.
The anti-nodal line is where
constructive interference occurs
(the waves that meet at those
points are in phase).
The nodal line is where
destructive interference occurs
(the waves that meet at those
points are out of phase).
Constructive interference displays maximum change in amplitudes
Destructive interference displays minimal or no change in amplitudes.
Chapter 17: Wave Phenomena
Constructive interference
S1
t
S2
t
(a) Waves from S1 and S2 arrive at point X in phase.
Each wave has amplitude of A. When they meet,
they combine to form resultant amplitude.
X
t
(b) The resultant amplitude to the two sources
fluctuates from 2A to (−2A).
Destructive interference
S1
t
S2
t
(a) Waves from S1 and S2 arrive at point Y out of
phase. Wave S1 has amplitude of A and Wave S2
has amplitude of (−A) at point Y. When they meet,
they combine to form resultant amplitude of zero.
Y
t
Chapter 17: Wave Phenomena
(b) The resultant amplitude of the two sources is
zero.
Young’s Double Slit Experiment
Thomas Young demonstrated the idea of interference using the double slit
experiment.
Two coherent light sources were used to produce an interference pattern.
Chapter 17: Wave Phenomena
• A monochromatic light source is
used.
• A small slit S3 is used to diffract light
and simulate a point source. Light
rays from S3 enter slits S1 and S2, and
simulate two more point sources.
• The diffracted waves from these two
simulated points overlap and
superimpose to form an interference
pattern.
Constructive interference occurs along the anti-nodal lines. In Young’s double slit
experiment, the path difference of the points along this line from the two sources is
equal to 1 λ, 2 λ, 3 λ and so on.
Destructive interference occurs along the nodal lines. In Young’s double slit
experiment, the path difference of the points along this line from the two sources is
equal to 0.5 λ, 1.5 λ, 2.5 λ and so on.
Chapter 17: Wave Phenomena
How slit separation and wavelength affects the interference fringe pattern:
(a) An interference fringe pattern
from a Young’s double slit
experiment. Note the positions
of the anti-nodal lines.
(b) More fringes appear when
the wavelength is decreased.
Note that the anti-nodal lines are
closer together.
(c) More fringes also appear
when the slit separation is
increased. Note that the antinodal lines are closer together.
Chapter 17: Wave Phenomena