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.

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.

Assessment A water wave moves from deep to shallow water. Describe changes that occur to the following characteristics of the wave as it crosses a boundary from deep to shallow water: wave speed wavelength frequency

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.

Physics terms crest trough wavefront propagation reflection refraction diffraction 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.

Representing waves 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. Demonstrate wave propagation with a Shive wave machine, if available.

Exploring the ideas 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.

Investigation Part 1: Investigate reflection Press [Run] to watch the waves propagate. Change wavelength and/or frequency. Press [Run] to see the new simulation. Repeat the simulation for three different boundaries: angled wall curved concave wall curved convex wall

Investigation Questions for Part 1 Sketch a plane wave reflecting from a straight wall. How does the wave direction change? Sketch a plane wave reflecting from an angled wall. How does the wave direction change? Sketch a plane wave reflecting from a concave wall and a convex wall.

Investigation Part 2: Investigate refraction, diffraction, and interference Investigate refraction of plane waves for flat and angled boundaries. Investigate diffraction of plane waves around a half wall, and through single and double gaps. Investigate diffraction by varying the wavelength for the single-gap wall.

Investigation Part 2: Investigate refraction, diffraction, and interference Investigate absorption using a flat boundary. Investigate interference using two circular waves. Answer the questions on your student assignment.

Reflection Reflection occurs for both longitudinal and transverse waves. Reflection causes a wave to change direction, and may also change its shape.

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.

Fixed boundaries A fixed boundary does NOT move in response to a wave. The wave pulse reflects on the opposite side of the spring. This behavior can be explained with Newton’s third law. The incoming pulse in the spring pulls UP on the wall, and the wall pulls DOWN on the spring.

Open boundaries 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.

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.

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.

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) Waves on the open ocean will refract as they enter the shallow water near the shore of a beach or island. The refraction will bend the wave crests so that they come in more parallel to the shoreline.

Refraction and direction Refraction changes the direction of a wave.

Refraction and direction Refraction changes the direction of a wave.

Refraction and wavelength Refraction also changes the wavelength of a wave.

Refraction and wavelength Refraction also changes the wavelength of a wave. Notice: as the wave slows down, its wavelength gets shorter.

Refraction and frequency Recall: When wave velocity changes during refraction, the wavelength also changes. But frequency CAN’T change: Every wave that enters the boundary must exit the boundary. Therefore, the number of waves per second must stay constant.

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.

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 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 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”. This image on the right from the simulation reveals the single slit interference pattern. Students will not be ready to understand this pattern, but should be able to see the brighter central maxima and shadow zones to each side.

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.

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

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 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.

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. absorption A magnifying glass enlarges an image. refraction Waves curve around a boulder in the water. diffraction A yell echoes off a building. reflection

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

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 The wave speed decreases. wavelength The wavelength decreases. frequency The wave frequency does NOT change.

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.

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

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. 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.