Wave interactions. Objectives Examine and describe wave propagation. Investigate behaviors of waves: reflection, refraction, and diffraction. Describe

$Page 1: Wave interactions. Objectives Examine and describe wave propagation. Investigate behaviors of waves: reflection, refraction, and diffraction. Describe$
Wave interactionsWave interactions

ObjectivesObjectives

• 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.

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

AssessmentAssessment

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.

2. A water wave moves from deep to shallow water.


a. wave speed

b. wavelength

a. frequency

Describe changes that occur to the following characteristics of the wave as it crosses a boundary from deep to shallow water:

AssessmentAssessment3. Wave behaviors and characteristics:

a. Describe the wave behavior that allows you to hear sound from another room through a crack in the door.

b. Describe the wave characteristic that makes radio transmission possible.

Physics termsPhysics terms

• crest

• trough

• wavefront

• propagation

• reflection

• refraction

• diffraction

• absorption

EquationsEquations

wave speed:

Describing wavesDescribing wavesA crest represents all the high points in a wave.

A trough is all the low points in the wave.

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.

Representing wavesRepresenting waves

Animated illustration, page 418

PropagationPropagationTo propagate is to spread out and grow.

Waves propagate outwards from their source, carrying both energy and information.

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.

How do waves propagate?How do waves propagate?

In Investigation 15B you will explore wave propagation and wave interactions in a simulated ripple tank.

The simulation displays wave behavior using a wavefront representation.

Exploring the ideasExploring the ideas

InvestigationInvestigationPart 1: Investigate reflection

1. Press [Run] to watch the waves propagate.

2. Change wavelength and/or frequency. Press [Run] to see the new simulation.

3. Repeat the simulation for three different boundaries:• angled wall• curved concave wall• curved convex wall

InvestigationInvestigationQuestions for Part 1

a. Sketch a plane wave reflecting from a straight wall. How does the wave direction change?

b. Sketch a plane wave reflecting from an angled wall. How does the wave direction change?

c. Sketch a plane wave reflecting from a concave wall and a convex wall.

InvestigationInvestigationPart 2: Investigate refraction, diffraction, and interference

1. Investigate refraction of plane waves for flat and angled boundaries.

2. Investigate diffraction of plane waves around a half wall, and through single and double gaps.

3. Investigate diffraction by varying the wavelength for

the single-gap wall.

InvestigationInvestigationPart 2: Investigate refraction, diffraction, and interference

4. Investigate absorption using a flat boundary.

5. Investigate interference using two circular waves.

Answer the questions on your student assignment.

ReflectionReflectionReflection occurs for both longitudinal and transverse waves.

Reflection causes a wave to change direction, and may also change its shape.

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.

BoundariesBoundaries

A fixed boundary does NOT move in response to a wave.

The wave pulse reflects on the opposite side of the spring.

Fixed boundariesFixed boundaries

Open boundariesOpen boundariesAn 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 alter both the shape and direction of a wavefront.

Curved boundariesCurved boundaries

•They can turn plane waves into circular waves that converge at a point.

•They can also change the curvature of a circular wave.

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.

Is reflection useful?Is reflection useful?

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.

RefractionRefraction

Waves move fast in deep water.

A-B moves slower in

shallow water.

A-C moves slower in

shallow water.

Refraction of a water waveRefraction of a water wave

Shallow (slow)

Refraction changes the direction of a wave.

Refraction and directionRefraction and direction

Refraction changes the direction of a wave.

Refraction and directionRefraction and direction

Refraction also changes the wavelength of a wave.

Refraction and wavelengthRefraction and wavelength

Refraction also changes the wavelength of a wave.

Notice: as the wave slows down, its wavelength gets shorter.

Refraction and wavelengthRefraction and wavelength

Recall:

When wave velocity changes during refraction, the wavelength also changes.

But frequency CAN’T change:

Refraction and frequencyRefraction and frequency

Every wave that enters the boundary must exit the boundary. Therefore, the number of waves per second must stay constant.

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.

All waves refractAll waves refract

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.

Is refraction useful?Is refraction useful?

Diffraction often changes the direction and shape of a wave.

DiffractionDiffractionDiffraction is a property of waves that allows them to bend around obstacles and pass through gaps.

DiffractionDiffractionLonger 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”.

you are here

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?

A paradoxA paradox

DiffractionDiffractionLonger 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.

Is diffraction useful?Is diffraction useful?

Radio waves have long wavelengths (10 to 1000 m long). This allows them to diffract around obstacles such as mountains.

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.

Diffraction in technologyDiffraction in technology



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.



a. Tarmac heats up on a sunny day. absorption

a. A magnifying glass enlarges an image. refraction

b. Waves curve around a boulder in the water. diffraction

c. A yell echoes off a building. reflection



a. wave speed

b. wavelength

a. frequency

Describe changes that occur to the following characteristics of the wave as it crosses the boundary from deep to shallow water:

a. wave speed The wave speed decreases.

b. wavelength The wavelength decreases.

a. frequency The wave frequency does NOT change.



Describe changes that occur to the following characteristics of the wave as it crosses the boundary from deep to shallow water:

3. Wave behaviors and characteristics:



a. Describe the wave characteristic that makes radio transmission possible.



AssessmentAssessment3. Wave behaviors and characteristics:

Diffraction causes sound waves to spread through a door, so you can hear from another room.




Diffraction causes sound waves to spread through a door, so you can hear from another room.

Radio waves have long wavelengths that allow them to bend (diffract) around obstacles.

3. Wave behaviors and characteristics:

Documents

Wave interactions. Objectives Examine and describe wave propagation. Investigate behaviors of waves: reflection, refraction, and diffraction. Describe