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Wave interactions
Objectives
•
Examine and describe wave propagation.
•
Investigate behaviors of waves: reflection, refraction,
and diffraction.
•
Describe the role of wave characteristics and behaviors
in medical and industrial applications.
Physics terms
•
crest
•
reflection
•
trough
•
refraction
•
wavefront
•
diffraction
•
propagation
•
absorption
Equations
wave speed:
Describing waves
A crest represents all the high
points in a wave.
A trough is all the low points in
the wave.
NOTES
Representing waves
NOTES
The crest of a wave is sometimes
called a wavefront.
In these figures, wavefronts are
shown in dark blue.
Waves propagate in a direction
perpendicular to their wavefronts.
Animated illustration, page 418
Propagation
To propagate is to spread out
and grow.
Waves propagate outwards
from their source, carrying both
energy and information.
How do waves propagate?
How do waves propagate?
Waves propagate because of connections between
the particles in the wave medium.
A disturbance in one place causes a disturbance in
the adjacent matter, such as in this water wave below.
NOTES
Reflection
Reflection occurs for both
longitudinal and transverse waves.
Reflection causes a wave to
change direction, and may also
change its shape.
NOTES
Boundaries
Reflection occurs at boundaries
where conditions change—such
as the edge of a pool or a wall in
a room.
The kind of reflection that occurs
depends on whether the
boundary is fixed or open.
NOTES
Fixed boundaries
A fixed boundary does NOT move in response to a wave.
The wave pulse reflects on the opposite side of the spring.
NOTES
Open boundaries
NOTES
An open boundary allows the end of the spring to move freely.
The wave reflects on the same side of the spring as the incident wave.
Curved boundaries
Curved boundaries alter both the
shape and direction of a wavefront.
• They can turn plane waves
into circular waves that
converge at a point.
• They can also change the
curvature of a circular wave.
NOTES
Is reflection useful?
Reflection is used in many technologies.
• Concave reflectors are employed
extensively in communications
technology such as satellite dish
receivers.
• This convex reflector provides an
expanded view for a bus driver.
• Concave reflectors are also used to
focus the headlights of cars.
NOTES
Refraction
Refraction occurs when a wave
changes speed at a boundary,
resulting in a change of direction.
Water waves refract if the depth
changes.
They refract because they move
slower in shallow water than in
deep water.
NOTES
Refraction of a water wave
Waves move fast
in deep water.
A-B moves
slower in
shallow water.
A-C moves
slower in
shallow water.
Shallow
(slow)
All waves refract
Refraction occurs for both transverse
and longitudinal waves.
• Light waves are transverse waves.
Light refracts when it changes speed
passing from air to water.
• Sound waves are longitudinal waves.
Sound refracts when it changes speed
passing from cool air into warm air.
NOTES
Is refraction useful?
Refraction is important in many
technologies:
• In optical systems such as
cameras, telescopes, and eye
glasses, lenses refract light waves.
• Ultrasound imaging detects
changes in tissue density by
reflecting AND refracting very high
frequency sound waves.
Diffraction
NOTES
Diffraction is a property of waves that allows them
to bend around obstacles and pass through gaps.
Diffraction often
changes the direction
and shape of a wave.
Diffraction
NOTES
Longer wavelengths = more bending.
When the wavelength is large
compared to the gap, the waves
diffract in complete arcs.
When the wavelength is small relative
to the gap, there is less diffraction and
a larger “shadow zone”.
A paradox
You are around the corner
from a lamp and a speaker.
Sound and light are both
waves, and both can diffract.
You can hear the speaker but
not see the lamp. Why?
you are
here
Diffraction
Longer wavelengths = more bending.
• Sound waves diffract around corners
because sound waves have long
wavelengths of centimeters to meters.
• Light waves also diffract, but their
wavelength is much smaller (~10-5 cm),
so the diffraction is imperceptibly small.
Light casts sharp shadows.
Diffraction in technology
Radio waves have long wavelengths
(10 to 1000 m long). This allows them to
diffract around obstacles such as mountains.
Cell phones use much shorter wavelengths
(6 – 12 cm), so cell phone transmissions
diffract (spread) less. You need line-of-sight
from the phone to the tower for transmission.
Assessment
1. Define the following events as fitting one of the wave-boundary
interactions. Use each term (reflection, refraction, absorption, and
diffraction) once.
a. Tarmac heats up on a sunny day.
b. A magnifying glass enlarges an image.
c. Waves curve around a boulder in the water.
d. A yell echoes off a building.
Assessment
2. A water wave moves from deep to shallow water.
Describe changes that occur to the following
characteristics of the wave as it crosses the
boundary from deep to shallow water:
a. wave speed
b. wavelength
a. frequency
Assessment
3. Wave behaviors and characteristics:
a. Describe the wave behavior that allows you to hear
sound from another room through a crack in the door.
a. Describe the wave characteristic that makes radio
transmission possible.