CHAPTER 14 WAVES

14.1 NATURE OF WAVES OBJECTIVES Relate waves and the transfer of energy. Distinguish between transverse waves and longitudinal waves. Predict the motion of a medium as a wave of energy passes through it. Infer the energy content of a longitudinal wave.

WHAT IS A WAVE? A wave is a disturbance that transfers energy through matter or through space. Some kinds of waves move through a medium, such as water or air. A medium is matter that is made of molecules and takes up space. A medium may be a solid, liquid, or gas.

WHAT IS A WAVE? cont. Other kinds of waves, such as light waves, can move through a vacuum. Light waves are unique in that they don't require a medium.

WAVES & ENERGY TRANSFER When energy waves move through a medium, the medium remains at the same location. You can see this occur if you throw a pebble near a leaf floating in water. As the wave of energy passes, the leaf bobs up and down, but not outward. This is because the water molecules beneath it remain in the same location. The water molecules simply transfer energy.

TYPES OF WAVES All waves don't have the same effect on a medium. The energy in heat waves and sound waves produces two different types of waves, called transverse waves and longitudinal waves. You can identify the type of wave passing through a medium by the way the medium is disturbed.

Transverse Waves Any wave in which the medium moves at right angles to the direction of the wave is a transverse wave. The crest is the highest point of a transverse wave. The trough is the lowest point of a transverse wave.

Longitudinal Waves Unlike a transverse wave, a longitudinal wave moves in the same direction as the medium. You can see longitudinal waves in comparison to transverse waves in the picture. Longitudinal waves move in the same direction as the disturbance in the medium.

Longitudinal Waves cont. The energy in the spring comes from compressing several coils to produce potential energy. When the compressed coils are released, the potential energy becomes kinetic energy, which moves through the spring like a wave. The part of a longitudinal wave where the particles of matter are close together is called a compression. Coils cannot be compressed in one part of a spring without causing coils to spread apart in another part of the spring. A rarefaction is the part of a longitudinal wave where the particles spread apart. **All longitudinal waves consist of alternating compressions and rarefactions.

Video Wave quiz-Brain pop https://www.brainpop.com/science/energy/waves/

Wave Motion Waves that move through the surface of a medium are called surface waves. Ocean surface waves are created by the force of friction when the wind moves across the water's surface to make ripples. The ripples expose more surface area for the wind to act upon; the small waves gain energy from the wind and increase in size.

SURFACE WAVES The individual water molecules in an ocean wave move in a circle. The size of the circle becomes smaller as the depth of the water increases. The decrease in circle size is due to the decreasing energy of the wave.

14.2 WAVES PROPERTIES OBJECTIVES Identify the parts of transverse and longitudinal waves. Calculate the wave speed if given the frequency and the wavelength. Predict how energy affects the amplitude of a wave. Compare and contrast wave height and amplitude.

What wave properties are seen as the waves approach the beach? Wave Behavior Even though waves often differ in size, speed, and shape, they all share common characteristics. Waves transfer energy. They have height, speed, length, & frequency (vary from wave to wave). What wave properties are seen as the waves approach the beach?

Characteristics of Waves Each kind of wave behaves differently in a medium. When a transverse wave passes through a medium, it looks like a moving snake. When a longitudinal wave passes through a medium, it looks more like a traveling earthworm. Compare the different parts of the transverse and longitudinal waves. Picture on page 342

Wavelength On a transverse wave, wavelength is the distance between two consecutive crests, or two consecutive troughs. The wavelength of a longitudinal wave is the distance between two consecutive compressions, or two consecutive rarefactions.

Wave Amplitude The amplitude of a transverse wave is the vertical distance between the line of origin and each crest or trough. The amplitude is related to how much energy the wave transfers through the medium. A wave with a high amplitude has more energy than a wave with a low amplitude.

Wave Frequency The frequency of a wave is the number of wavelengths that pass a point in a given amount of time, such as a second. The unit for frequency is hertz (Hz). One wave per second equals 1 Hz. Figure 14.8 on page 344

speed = frequency x wavelength Wave Speed The frequency and wavelength determine the speed of a wave. Waves don't change speed as they travel through a medium. The speed of a wave is equal to the frequency times the wavelength. speed = frequency x wavelength The speed of the wave is measured in meters per second.

14.3 Wave Interactions OBJECTIVES Describe four kinds of wave interactions. Explain the relationship between the angle of incidence and the angle of reflection. Compare and contrast constructive and destructive interference. Predict how a wave will behave if it moves from air into water.

Reflection Reflection occurs when a wave bounces off a surface. The energy carried by reflected waves depends on how much energy is absorbed by the surface that the wave hits. The energy not absorbed by the surface is carried by the reflected waves.

Diffraction The bending of a wave as a result of the interaction between a wave and the edge of an object is called diffraction. The waves bend around the angular rock, but the wavelength remains unchanged. Notice how the energy of waves erodes the shoreline. Picture on page 350

Picture on page 351 (figure 14.13) Refraction Waves can change as they pass from one medium to another. A wave entering a new medium at an angle changes direction. The bending of a wave as a result of a change in speed is called refraction. https://www.brainpop.com/science/energy/refractionanddiffraction/ In air, the light waves move in a straight line, but when they enter a new medium, they bend. Picture on page 351 (figure 14.13)

Interference Waves change when they interact with each other. The effect of two or more waves interacting is called interference. The interaction between two waves when they meet may produce a large single wave or no wave at all.

What happens when 2 wave crests meet? A single wave forms as the two crests begin to overlap. When two crests are at the same location at the same time, constructive interference takes place. A single wave with a crest of maximum amplitude is produced. The amplitude of the wave formed is the sum of the amplitudes of the interfering crests.

What happens when a crest of one wave meets a trough of another wave? The crest and trough subtract from each other to form a single wave, or possibly, no wave. Destructive interference takes place, and produces water waves with a reduced amplitude. If the amplitudes of the crest arid the trough were equal, the waves would cancel out one another.