Chapter 10: Waves 

Section 1: The Nature of Waves

• What’s in a wave?
  • Wave: a repeating disturbance or movement that transfers energy through matter or space.
  • Ocean waves disturb the water and transfer energy through it.
  • During earthquakes, energy is transferred in powerful waves that travel through Earth.
  • Light is a type of wave that can travel through empty space to transfer energy from one place to another, such as from the Sun to Earth.
• Waves and Energy
  • Falling pebbles transfer their kinetic energy to the particles of water in a pond, forming waves.
  • All waves have the property to carry energy without transporting matter from place to place.
  • A wave will travel only as long as it has energy to carry.
  • Anything that moves up and down or back and forth in a rhythmic way is vibrating.
• Mechanical Waves
  • Sound waves travel through the air to reach your ears.
  • Ocean waves move through water to reach the shore.
  • Medium: the matter the waves travel through
  • The medium can be a solid, a liquid, a gas, or a combination of these.
  • Not all waves need a medium.
  • The two types of mechanical waves are transverse waves and compressional waves.
  • 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.
  • Compressional waves also are called longitudinal waves.
  • Sound waves are compressional waves.
  • When a sound wave reaches your ear, it causes your eardrum to vibrate.
  • Water waves are not purely transverse waves.
  • 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.
  • Seismic waves are a combination of compressional and transverse waves.
  • When objects on Earth’s surface absorb some of the energy carried 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 carry and in how fast they travel.
  • Transverse and compressional waves have different features that travel through a medium and form the wave.
  • Crests: high points of a wave
  • Troughs: low points of a wave
  • When a compressional wave passes through a medium, it creates regions where the medium becomes crowded together and more dense.
  • Rarefaction: less-dense region of a compressional wave
  • One wavelength starts at any point on a wave and ends at the nearest point just like it.
• Wavelength: the distance between one point on a wave and the nearest point just like it.
  • A wavelength in a compressional wave is the distance between two neighboring compressions or two neighboring rarefactions
  • You can measure from the start of one compression to the start of the next compression or from the start of one rarefaction to the start of the next rarefaction.
• Frequency and Period
  • When you tune your radio to a station, you are choosing radio waves of a certain frequency.
  • 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.
  • Frequency is expressed in hertz (Hz)
  • Period: the amount of time it takes one wavelength to pass a point.
  • As the frequency of a wave increases, the period decreases.
  • Period has units of seconds.
  • 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.
  • However, light waves travel more slowly in liquids and solids than they do in gases or in empty space.
  • You can calculate the speed of a wave represented by v by multiplying its frequency times its wavelength.
• Amplitude and Energy
  • Amplitude: related to the energy carried by a wave.
  • The greater the wave’s amplitude is, the more energy the wave carries.
  • 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 carries.
  • The closer the coils are in a compression, the farther apart they are in a rarefaction.
  • The amplitude of any transverse wave is the distance from the crest or trough of the wave to the rest position of the medium

Section 3: The Behavior of Waves

• Reflection

  • Reflection occurs when a wave strikes an object and bounces off of it.
  • All types of waves—including sound, water, and light waves— can be reflected.
  • Sometimes when the sound waves hit another object, they reflect off it and come back to you.
  • Bats and dolphins use echoes to learn about their surroundings.
  • The beam striking a mirror is called the incident beam.
  • The beam that bounces off the mirror is called the reflected beam.
  • The line drawn perpendicular to the surface of the mirror is called the normal.
  • According to the law of reflection, the angle of incidence is equal to the angle of reflection. All reflected waves obey this law.
  • Objects that bounce from a surface sometimes behave like waves that are reflected from a surface.

• Refraction: he 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 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.
  • The greater the change in speed is, the more the wave bends.
  • A flashlight beam is made of light waves. When any wave is reflected, the angle of incidence, i, equals the angle of reflection, r.
  • Light waves travel slower 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.

  • When waves strike an object, several things can happen.

  • The waves can bounce off, or be reflected.

  • Sometimes the waves may be both reflected and refracted.

  • Waves also can behave another way when they strike an object.

  • The waves can bend around the object.

  • When water waves pass through a small opening in a barrier, they diffract and spread out after they pass through the hole.

  • 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 an obstacle is smaller than the wavelength, the waves bend around it.

  • If the obstacle is much larger than the wavelength, almost no diffraction occurs.

  • Light waves have a much shorter wavelength.

  • Diffraction also affects your radio’s reception.

  • AM radio waves have longer wavelengths than FM radio waves do.

  • 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.
  • The two ways that the waves can combine are called constructive interference and destructive interference.
  • In constructive interference the waves add together.
  • Constructive interference also occurs when the compressions of different compressional waves overlap
  • Waves undergoing constructive interference are said to be in phase.
  • In destructive interference, the waves subtract from each other as they overlap.
  • With compressional waves, destructive interference occurs when the compression of one wave overlaps with the rarefaction of another wave.
  • Waves undergoing destructive interference are said to be out of phase.

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

  • At any point where a crest meets a crest, a new wave with a larger amplitude forms. But at points where crests meet troughs, the waves cancel each other and no motion occurs.

• 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.
  • All objects have their own natural frequencies of vibration that depend on the object’s size, shape, and the material it is made from.
  • 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.

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