Chapter 10: Waves 

Section 1: The Nature of Waves

• Wave: a repeating disturbance or movement that transfers energy through matter or space.

• Waves and Energy

  • Falling pebbles transfer their kinetic energy to the particles of water in a pond, forming waves.
  • All waves transfer energy without trans- porting matter from place to place.
  • A wave will travel only as long as it has energy to transfer.
  • Anything that moves up and down or back and forth in a rhythmic way is vibrating.
  • All waves are produced by something that vibrates.

• Mechanical Waves: Waves that can travel only through matter

  • Sound waves travel through the air to reach your ears.
  • Medium: can be a solid, a liquid, a gas, or a combination of these.
  • For sound waves, the medium is air, and for ocean waves, the medium is water. Not all waves need a medium. Some waves, such as light and radio waves, can travel through space.
  • Transverse Wave: matter in the medium moves back and forth at right angles to the direction that the wave travels.
  • A water wave travels horizontally as the water moves vertically up and down.
  • Compressional Wave: matter in the medium moves back and forth along the same direction that the wave travels
  • In a compressional wave in a coiled- spring toy, the wave travels horizontally along the spring, and the coils in the spring move back and forth horizontally.
  • Sound waves are compressional waves.
    • Sound waves also can travel through other mediums, such as water and wood.
  • A water wave causes water to move back and forth, as well as up and down. Water is pushed back and forth to form the crests and troughs.
  • Ocean waves are formed most often by wind blowing across the ocean surface.
  • When Earth’s crust shifts or breaks, the energy that is released is transmitted outward, causing an earthquake.
  • When objects on Earth’s surface absorb some of the energy transferred by seismic waves, they move and shake. The more the crust moves during an earthquake, the more

  energy is released.

Section 2: Wave Properties

• The Parts of a Wave
  • Waves can differ in how much energy they transfer and in how fast they travel.
  • Crest: highpoint of a wave
  • Trough: Low-point of a wave
  • Transverse and compressional waves have different features that travel through a medium and form the wave.
  • A compressional wave has no crests and troughs.
  • Rarefactions: less-dense regions of a compressional wave
• Wavelength: the distance between one point on a wave and the nearest point just like it.
  • One wavelength starts at any point on a wave and ends at the nearest point just like it.
  • For transverse waves, a wavelength can be measured from crest to crest or trough to trough.
  • The wavelength of a compressional wave can be measured from compression to compression or from rarefaction to rarefaction.
• Frequency and Period
  • Frequency: the number of wavelengths that pass a fixed point each second.
  • You can find the frequency of a transverse wave by counting the number of crests or troughs that pass by a point each second.
  • The frequency of a compressional wave is the number of compressions or rarefactions that pass a point every second.
  • Frequency is expressed in hertz (Hz). A frequency of 1 Hz means that one wavelength passes by in 1 s.
  • Period: the amount of time it takes one wavelength to pass a point.
  • Periods are measured in units of seconds.
  • The wavelength of a wave decreases as the frequency increases.
  • The frequency of a wave is always equal to the rate of vibration of the source that creates it.
• Wave Speed
  • The speed of a wave depends on the medium it is traveling through.
  • Sound waves usually travel faster in liquids and solids than they do in gases
  • Light waves travel more slowly in liquids and solids than they do in gases or in empty space.
  • Wave Speed Equation: speed (in m/s) = frequency (in Hz) X wavelength (in m)
• Amplitude and Energy
  • Amplitude: related to the energy transferred by a wave.
  • The greater the wave’s amplitude, the more energy the wave transfers
  • Amplitude is measured differently for compressional and transverse waves.
  • The amplitude of a compressional wave is related to how tightly the medium is pushed together at the compressions.
  • The denser the medium is at the compressions, the larger its amplitude is and the more energy the wave transfers.
  • The amplitude of a transverse wave is the distance between a crest or a trough and the position of the medium at rest.

Section 3: The Behavior of Waves

• Reflection: occurs when a wave strikes an object and bounces off it.

  • All types of waves—including sound, water, and light waves— can be reflected
  • Sound waves form when your foot hits the floor and the waves travel through the air to both your ears and other objects.
  • Bats and dolphins use echoes to learn about their surroundings. A dolphin makes a clicking sound and listens to the echoes. These echoes enable the dolphin to locate nearby objects.
  • A flashlight beam is made of light waves. When any wave is reflected, the angle of incidence, i, equals the angle of reflection, r.

• Refraction: the bending of a wave caused by a change in its speed as it moves from one medium to another.

  • When a wave passes from one medium to another—such as when a light wave passes from air to water— it changes speed
  • If the wave is traveling at an angle when it passes from one medium to another, it changes direction, or bends, as it changes speed.
  • Light waves travel more slowly in water than in air. This causes light waves to change direction when they move from water to air or air to water.

• Diffraction: occurs when an object causes a wave to change direction and bend around it.

  • Diffraction and refraction both cause waves to bend. The difference is that refraction occurs when waves pass through an object, while diffraction occurs when waves pass

  around an object.

  • When water waves pass through a small opening in a barrier, they diffract and spread out after they pass through the hole.
  • If the obstacle is much larger than the wavelength, almost no diffraction occurs
  • Diffraction also affects your radio’s reception.
  • The diffraction of waves around an obstacle depends on the wavelength and the size of the obstacle.

• Interference: When two or more waves overlap and combine to form a new wave

  • Interference occurs while two waves are overlapping. Then the waves combine to form a new wave. Two waves traveling on a rope can interfere with each other.
  • When waves interfere with each other, constructive and destructive interference can occur. Infer how the energy transferred
  • In constructive interference the waves add together
  • The amplitude of the new wave that forms is equal to the sum of the amplitudes of the original waves.
  • Constructive interference also occurs when the compressions of different compressional waves overlap.

• Standing Wave: a special type of wave pattern that forms when waves equal in wavelength and amplitude, but traveling in opposite directions, continuously interfere with each other.

  • The places where the two waves always cancel are called nodes.
  • The wave pattern vibrates between the nodes.

• Resonance: The process by which an object is made to vibrate by absorbing energy at its natural frequencies

  • When you strike a bell, the bell vibrates at certain frequencies called the natural frequencies.
  • Sometimes, resonance can cause an object to absorb a large amount of energy.
  • An object vibrates more strongly as it continues to absorb energy at its natural frequencies. If enough energy is absorbed, the object can vibrate so strongly that it breaks apart.

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