Ch. 11.3 Wave Interactions
Section 11.3 Objectives

Describe how waves behave when they
meet an obstacle, pass into another
medium, or pass through another wave.

Explain what happens when two waves
interfere.

Distinguish between constructive
interference and destructive interference.

Explain how standing waves are formed.
Reflection

Reflection is the
bouncing back of a
wave as it meets a
surface or boundary.
 At a free boundary,
waves are reflected.
 At a fixed boundary,
waves are reflected
and inverted.
Diffraction

Diffraction is the
bending of a wave as it
passes an edge or an
opening.

When waves pass the
edge of an object or
pass through an
opening, such as a
door, they spread out
as if a new wave were
created there.
Refraction

Refraction is the
bending of waves as
they pass from one
medium to another.

All waves are
refracted when they
pass from one
medium to another
at an angle.
Because light waves bend when they
pass from one medium to another,
the straws look like they are in two
pieces.
Wave Interference

Interference is the
combination of two or
more waves that exist in
the same place at the
same time.

Two different material
objects can never
occupy the same space
at the same time.

Waves, however, can
pass through one
another.

When this happens, the
waves form an
interference pattern.
This ripple tank demonstrates the
interference of water waves.
Types of Interference

Constructive
interference –
displacements in the
same direction.

The method of
summing the
displacements of
waves is known as the
superposition principle.
When these two wave pulses meet,
the displacements at each point
add up to form a resultant wave.
This is an example of constructive
interference.
Types of Interference

Destructive
Interference –
displacements in the
opposite directions.

Complete Destructive
Interference – two
pulses equal in
amplitude but opposite
in direction, they
completely cancel
each other.
In this case, known as destructive
interference, the displacement of
one pulse is subtracted from the
displacement of the other.
Interference of Light waves

Interference of light
waves creates
colorful displays.

You can see a
rainbow of colors
when oil is spilled
onto a watery
surface.
Spilt oil on
watery asphalt

These colors are due
to interference of
light.
Beats

Sound waves of slightly different frequencies will
interfere with each other and produce loud and
soft sounds called beats.
Sound waves at slightly different frequencies
produce beats.
 The number of beats per second corresponds to
the difference between frequencies.

Standing Waves

Standing wave is a wave
from caused by interference
that appears not to move
along the medium and that
shows some regions of no
vibration (nodes) and other
regions of maximum
vibration (antinodes).

The points at which two
waves cancel are nodes.

Between two adjacent
nodes where the amplitude
is the greatest, these points
are called anti nodes.
Section 11.3 Summary
Waves bouncing off a surface is called
reflection.
 Diffraction is the bending of waves as they
pass from one medium to another.
 Refraction is the bending of waves as they
pass from one medium to another.
 Interference results when two waves exist in
the same place and combine to make a
single wave.
 Interference may cause standing waves.
