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.