Chapter 20

Chapter 20Waves

Section 1: Objectives• Describe how waves transfer energy

without transferring matter.

• Distinguish between waves that require a medium and waves that do not.

• Explain the difference between transverse and longitudinal waves.

Wave Energy• A wave is any disturbance that

transmits energy through matter or empty space.

• Energy can be carried away from its source by a wave.

• The material through which the wave travels does not move with the energy.

Wave Energy

Wave Energy• As a wave travels, it does work on everything

in its path.

• The waves in a pond do work on the water to make it move up and down. • The waves also do work on anything floating on the

water’s surface.

• The fact that the water and floating objects move tells you that the waves are transferring energy.

Wave Energy• Most waves transfer energy by the

vibration of particles in a medium.

• A medium is a substance through which a wave can travel.

• Sound waves, water waves, and seismic waves all need a medium through which to travel.

Wave Energy• Visible light waves, microwaves, radio

waves, and X rays are examples of waves that can transfer energy without going through a medium.

• These waves are electromagnetic waves.

• Electromagnetic waves do not require a medium like other waves.

Wave Energy• Transverse Waves are waves in which

the particles vibrate perpendicularly to the direction the wave is traveling.

• Transverse waves are made up of crests and troughs.

• Water waves, waves on a rope, and electromagnetic waves are examples of transverse waves.

Wave Energy• Longitudinal Waves are waves in which

the particles vibrate back and forth along the path that the waves moves.

• Longitudinal waves are made up of compressions and rarefactions.

• Waves on a spring are longitudinal waves.

Wave Energy

Wave EnergySound Waves are longitudinal waves.

Sound waves travel by compressions and rarefactions of air particles, as shown below.

Wave Energy• Combinations of Waves A

transverse waves and a longitudinal wave can combine to form a surface wave.

• Surface waves look like transverse waves, but the particles of the medium move in circles rather than up and down.

Wave Energy

Chapter 20 Sec. 1 Pop Quiz1) T/F Energy can’t be carried away

from its source by a wave.2) T/F As a wave travels it does work

on the path.3) How do most waves transfer energy?4) List 1 type of wave that does not

require a medium to pass through.5) What is the difference between

transverse and longitudinal waves?6) What is a surface wave?

Section 2: Objectives• Identify and describe four wave

properties.

• Explain how frequency and wavelength are related to the speed of a wave.

Parts Of A WaveThe amplitude of a wave is the

maximum distance that the particles of a medium vibrate from their rest position.

A wave with a large amplitude carries more energy than a wave with a small amplitude.

Parts Of A Wave• A wavelength is the distance between

any point on a wave to an identical point on the next wave.

• A wave with a shorter wavelength carries more energy than a wave with a longer wavelength does.

Parts Of A Wave• Frequency is the number of waves

produced in a given amount of time.

• Frequency is usually expressed in hertz (Hz). • One hertz equals one wave per second.

• If the amplitudes are equal, high-frequency waves carry more energy than low-frequency waves.

Parts Of A Wave

Parts Of A Wave• Wave Speed is the speed at which a

wave travels.

• Wave speed (v) can be calculated using wavelength () and frequency (f), by using the wave equation, which is shown below:

•v f

Calculating Wavelength

Example # 1What is the frequency of a wave

if the wave has a speed of 12 cm/s and a wavelength of 3 cm?

Remember: v f = v / f (Rearranged by dividing by f)

F = V / (Rearranged by dividing by )

Example # 2A wave has a frequency of 5 Hz

and a wave speed of 18 m/s. What is the wavelength?

Remember: v f = v / f (Rearranged by dividing by f)

F = V / (Rearranged by dividing by )

Parts of A Wave• Frequency and wavelength are inversely

related. • So, if one value is doubled, the other value

will be cut in half.

• The wave speed of a wave in a certain medium is the same no matter what the wavelength is. • So, the wavelength and frequency depend on

the wave speed, not the other way around.

Chapter 20 Sec. 2 Pop Quiz1) What is the unit used to measure

frequency?2) What is 1 Hz equal to?3) What is the formula used to

calculate wave speed?4) How are frequency and

wavelength related?5) What will happen to the speed of

a wave in a certain medium if the wavelength changes?

Section 3: Objectives• Describe reflection, refraction,

diffraction, and interference.

• Compare destructive interference with constructive interference.

• Describe resonance, and give examples.

Wave Angles• Reflection happens when a wave bounces

back after hitting a barrier.

• Light waves reflecting off an object allow you to see that object. • A reflected sound wave is called an echo.

• Waves are not always reflected when they hit a barrier. • A wave is transmitted through a substance when it

passes through the substance.

Wave Angles• Refraction is the bending of a wave

as the wave pass from one medium to another at an angle.

• When a wave moves from one medium to another, the wave’s speed and wavelength changes. • As a result, the wave bends and travels in

a new direction.

Wave Angles• Diffraction is the bending of waves

around a barrier or through an opening.

• The amount of diffraction depends on its wavelength and the size of the barrier or opening the wave encounters.

Wave Angles• Interference is the result of two or more

waves overlapping.

• Constructive Interference happens with the crests of one wave overlap with the crests of another wave or waves. The troughs of the waves also overlap.

• Constructive Interference results in a new wave that has a larger amplitude than the original waves had.

Wave Angles• Destructive Interference happens with the crests

of one wave and the troughs of another wave overlap.

• Destructive interference results in a new wave that has a smaller amplitude than the original waves had.

• When the waves involved in destructive interference have the same amplitude and meet each other at just the right time, the result is no wave at all.

Wave Angles

Wave Angles• Standing Waves are waves that appear to be

standing still.

• A standing wave only looks as if it is standing still. • Waves are actually going in both directions.

• In a standing wave, certain parts of the wave are always at the rest position because of total destructive interference. • Other parts have a large amplitude because of

constructive interference.

Wave Angles• The frequencies at which standing

waves form are called resonant frequencies.

• Resonance happens when an object vibrating at or near the resonant frequency of a second object causes the second object to vibrate.