Wave interactions

1
Wave interactions
2
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.

3
Physics terms

• crest
• trough
• wavefront
• propagation

• reflection
• refraction
• diffraction
• absorption

4
Equations
wave speed
5
Describing waves
NOTES
A crest represents all the high points in a
wave. A trough is all the low points in the
wave.
6
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
7
Propagation
To propagate is to spread out and grow. Waves
propagate outwards from their source, carrying
both energy and information. How do waves
propagate?
8
How do waves propagate?
NOTES
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.
9
Reflection
NOTES
Reflection occurs for both longitudinal and
transverse waves. Reflection causes a wave to
change direction, and may also change its
shape.
10
Boundaries
NOTES
Reflection occurs at boundaries where conditions
changesuch 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.
11
Fixed boundaries
NOTES
A fixed boundary does NOT move in response to a
wave. The wave pulse reflects on the opposite
side of the spring.
12
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.
13
Curved boundaries
NOTES
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.

14
Is reflection useful?
NOTES

• 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.

15
Refraction
NOTES
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.
16
Refraction of a water wave
A-B moves slower in shallow water.
A-C moves slower in shallow water.
Waves move fast in deep water.
Shallow (slow)
17
All waves refract
NOTES

• 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.

18
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.

19
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.
20
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.
21
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
22
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.

23
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.
24
Assessment

• Define the following events as fitting one of the
  wave-boundary interactions. Use each term
  (reflection, refraction, absorption, and
  diffraction) once.

• Tarmac heats up on a sunny day.
• A magnifying glass enlarges an image.
• Waves curve around a boulder in the water.
• A yell echoes off a building.

25
Assessment

• 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

• wave speed
• wavelength
• frequency

26
Assessment

• Wave behaviors and characteristics

• Describe the wave behavior that allows you to
  hear sound from another room through a crack in
  the door.
• Describe the wave characteristic that makes radio
  transmission possible.