Nature of
waves
Starter
Look at the video:
Can you identify areas of “calm” water?
What may have caused this?
What may be occurring at other areas in the tank?
Learning Objectives
• Know what reinforcement and
cancellation of waves means.
• Know a demonstration to show
interference.
• Know what constructive and destructive
interference is, and when it will occur.
Success criteria
• Label areas of reinforcement and
cancellation.
(Grade C)
• Describe a demonstration which
shows reinforcement and
cancellation.
(Grade C)
• Explain why constructive or
destructive interference occurs.
(Grade A)
Interfering waves
• What will happen if the crests of two
identical waves meet?
– The wave height will double. This is called
REINFORCEMENT.
• What will happen if the crest of one
wave meets the trough of another
identical wave?
– The waves will subtract from each other,
giving calm water. This is CANCELLATION.
Interfering waves
• On the diagram given:
– put an R on areas of reinforcement.
– put an C on areas of cancellation.
Demonstration of interference
using sound waves
1. Take part in the demonstration.
2. Write a brief explanation of how the
demonstration works, what you
heard and why. Include a diagram
to aid your explanation. (Page 197,
fig 3. – old course).
• To produce a stable interference
pattern, the wave sources must be
coherent (must have the same
frequency and therefore
wavelength).
Task
• Foundation: complete worksheet
P5f2 – old course, then answer
questions 3 and 4 on page 197.
• Higher: you will need to work on
explaining interference patterns on
the next few slides.
• When two waves from the same
source meet in step (or in phase)...

...constructive interference occurs.
• When two waves from the same source
meet out of step (or in antiphase)...

...destructive interference occurs.
Constructive or destructive?
• Constructive interference occurs
when the path difference from the
two sources is a whole number of
wavelengths.
• Destructive interference occurs when
the path difference from the two
sources is an odd number of half
wavelengths.
LIGHT SOURCE
Imagine a light
source pointing at a
screen with two slits
in it, and projecting
onto a second
screen. (the two
light waves are
given a different
colour to make it
easier to see)
LIGHT SOURCE
Some of the waves
will bend as they
pass through the
slits. Why?
LIGHT SOURCE
Why are the waves in step?
Where diffracted waves meet
and
Because, inup,
this constructive
case, they have
travelled
destructive
interference
the same
distance (count
the peaks!).
occurs.
A
Waves are in step. There
will be a bright spot at
this point (Point A)
LIGHT SOURCE
There will be other
bright points apart
B
from A.
A
Why are the waves in step?
Waves are in step. There
The difference in distance
two waves
will bethe
a bright
spot at
have travelled is a whole
number
ofB)
this point
(Point
wavelengths (count them!)
Why are the waves out of step?
Destructive interference can also
The difference in distance the two waves have
occur.
travelled is an odd number
of half wavelengths
(count them!). i.e. Troughs and crests line up.
LIGHT SOURCE
B
C
A
Waves are out of step.
This point (Point C)will be
unlit.
Look at figure 6 on page 197 –
old course
• Make a note of the path difference
equations at the bottom of the page.
Tasks
• Foundation: answer questions 1
and 2 on worksheet P5f1 – old
course.
• Higher: answer questions 1 and 3
on worksheet P5f1 – old course.
Answers
1. Alternate loud and quiet areas of sound; at some points the sound waves
reinforce and at others they subtract from each other
Sound heard is an octave higher in pitch; the areas of loud and quiet
sound are closer together or have half the separation of in a
2. Bright light
No light (or darkness)
3. Waves must diffract to overlap; this requires a gap of similar size to the
wavelength; light has a very small wavelength (400–600 nm)
Path lengths from each slit to P differ by exactly one wavelength so waves
interfere constructively (crests or troughs arrive together)
Z; path lengths from each slit to Q and Z differ by exactly two
wavelengths
At M – darkness (or no light); path lengths from each slit to M differ by
exactly one and a half wavelengths so waves interfere destructively (crest
and trough meet)
Increase
Decrease
Learning Objectives
• Know what reinforcement and
cancellation of waves means.
• Know a demonstration to show
interference.
• Know what constructive and destructive
interference is, and when it will occur.
Success criteria
• Label areas of reinforcement and
cancellation.
(Grade C)
• Describe a demonstration which
shows reinforcement and
cancellation.
(Grade C)
• Explain why constructive or
destructive interference occurs.
(Grade A)
Plenary
Can you explain this? (reward
point for correct explanation)