Writing Frame –Interference of waves
The principle of superposition of waves states that when two or more waves are incident on the same
point, the total displacement at that point is equal to the vector sum of the displacements of the individual
waves. If a crest of a wave meets a crest of another wave of the same frequency at the same point, then
the magnitude of the displacement is the sum of the individual magnitudes – this is constructive
interference. If a crest of one wave meets a trough of another wave then the magnitude of the
displacements is equal to the difference in the individual magnitudes – this is known as destructive
interference.
combine
d
wavefor
m
wave 1
wave 2
Constructive interference
Destructive interference
Constructive interference occurs when the phase difference between the waves is a multiple of 2π,
whereas destructive interference occurs when the difference is π, 3π, 5π, etc. If the difference between the
phases is intermediate between these two extremes, then the magnitude of the displacement of the
summed waves lies between the minimum and maximum values.
Consider, for example, what happens when two identical stones are dropped into a still pool of water at
different locations. Each stone generates a circular wave propagating outwards from the point where the
stone was dropped. When the two waves overlap, the net displacement at a particular point is the sum of
the displacements of the individual waves. At some points, these will be in phase, and will produce a
maximum displacement. In other places, the waves will be in anti-phase, and there will be no net
displacement at these points.
You may draw a labelled diagram(s) in the box provided to support your answer.
Structure
Useful Starters
Opening Paragraph
The explanation begins with an
introduction to the topic using
What, when, where, why and who
When two or more waves pass through each other,
the displacements of the medium caused by each
individual wave combine to make a resultant wave.
This is known as interference.
Paragraph one
A description of the parts of the
topic
Constructive and destructive interference occurs
This focuses on the “what” ideas
Following Paragraphs
Each part is backed up by
supporting information, giving
the "how" or "why" details.
Describes “cause and effect”
using
Providing the waves have the same frequency and
when a crest meets a crest then constructive
interference occurs because the waves are meeting in
phase. If a trough meets a crest then destructive
interference occurs because the waves meeting are
out of phase usually by half a cycle. Since there are
Vocabulary
(specific to subject)
if, then, because, since, so
Time relationships are used first,
then, following, finally.
times when the waves are constructing together then
this causes antinodal regions generating a wave of
maximum amplitude.
The parts and paragraphs are in
a logical order
Summary
The summary includes the main
points and makes an interesting
comment,
Interference occurs in both sound and light. In
interference in white light dark patches where
destructive interference exists. And with constructive
interference bright regions are seen. With sound
regions of no sound and amplified sound are heard
along the destructive and constructive regions.
Technical terms