Waves

What’s in a wave?

• A wave is a repeating disturbance or movement that transfers energy through matter (medium) or space.

Waves and Matter

• All waves have this propertythey carry energy without transporting matter from place to place. – only energy moves forward

• A wave will travel only as long as it has energy to carry.

Making Waves

• Suppose you are holding a rope at one end, and you give it a shake.

• You would create a pulse that would travel along the rope to the other end, and then the rope would be still again.

Making Waves

• It is the up-and-down motion of your hand that creates the wave.

• Anything that moves up and down or back and forth in a rhythmic way is vibrating.

• The vibrating movement of your hand at the end of the rope created the wave. In fact, all waves are produced by something that vibrates.

Mechanical Waves • The matter the waves travel through is called a

medium. The medium can be a solid, a liquid, a gas, or a combination of these.

• Some waves, such as light and radio waves do no need a medium they can travel through space.

• Waves that can travel only through matter are called mechanical waves.

• The two types of mechanical waves are transverse waves and compressional waves.

Transverse Waves • Matter in the medium moves back and

forth at right angles to the direction that the wave travels.

• For example, a water wave travels horizontally as the water moves vertically up and down.

Compressional Waves • In a compressional wave, matter in the

medium moves back and forth along the same direction that the wave travels.

Compressional Waves

• As the wave moves, it looks as if the whole spring is moving toward one end.

• The wave carries energy, but not matter, forward along the spring.

• Compressional waves also are called longitudinal waves.

Sound Waves

• Sound waves are compressional waves. • When a noise is made, such as when a

locker door slams shut and vibrates, nearby air molecules are pushed together by the vibrations.

Sound in Other Materials

• Sound waves travel through any type of medium such as a Solid liquid or gas. (water or wood)

• Sound waves cannot travel through empty space

• When a sound wave reaches your ear, it causes your eardrum to vibrate.

• Your inner ear then sends signals to your brain, and your brain interprets the signals as sound.

The Speed of Sound in Different Materials

The speed of a sound wave through a medium depends on the substance the medium is made of and whether it is solid, liquid, or gas.

Speed doesn’t depend on loudness of sound

Solids = fastest timeGases = Slowest time

Water Waves

• Water waves are not purely transverse waves.

• A water wave causes water to move back and forth, as well as up and down.

Water Waves

• Ocean waves are formed most often by wind blowing across the ocean surface.

• The size of the waves that are formed depend on the wind speed, the distance over which the wind blows, and how long the wind blows.

Seismic Waves

• Forces in Earth’s crust can cause regions of the crust to shift, bend, or even break.

• The breaking crust vibrates, creating seismic (SIZE mihk) waves that carry energy outward.

Seismic Waves

• Seismic waves are a combination of compressional and transverse waves. They can travel through Earth and along Earth’s surface.

• The more the crust moves during an earthquake, the more energy is released. Click image to view movie

Parts and components of a wave

• Crest • Trough• Compressions • Rarefactions• Wavelength• Freqency• amplitude

The Parts of a Wave

• A transverse wave has alternating high points, called crests, and low points, called troughs.

The Parts of a Wave

• Compressional wave has no crests and troughs.

• A compression is a region where the coils are close together.

• Less dense regions (spread apart) = rarefaction

Wavelength • A wavelength is the distance between one

point on a wave and the nearest point just like it.

• For transverse waves the wavelength is the distance from crest to crest or trough to trough.

Wavelength

• A wavelength in a compressional wave is the distance between two neighboring compressions or two neighboring rarefactions.

Frequency and Period

• The frequency of a wave is 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).• Frequency relates to pitch = how high or

low a sound seems to be

Frequency and Period

• The period of a wave is 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.

Wavelength is Related to Frequency

• As frequency increases, wavelength decreases. • The frequency of a wave is always equal to the rate of vibration of the source that creates it.

• If you move the rope up, down, and back up in 1 s, the frequency of the wave you generate is 1 Hz.

• 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 liquid and solids than they do in gases or in empty space.

• Sound waves usually travel faster in a material if the temperature of the material is increased.

Wavelength is Related to Frequency

Calculating Wave Speed

• You can calculate the speed of a wave represented by v by multiplying its frequency times its wavelength.

Amplitude and Energy

• Amplitude is related to the energy carried by a wave.

• The greater the wave’s amplitude is, the more energy the wave carries.

• Amplitude is measured differently for compressional and transverse waves.

Click image to play movie

Amplitude of Compressional Waves

• The amplitude of a compressional wave is related to how tightly the medium is pushed together at the compressions.

Amplitude of Compressional Waves

• 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.

• So the less dense the medium is at the rarefactions, the more energy the wave carries.

Amplitude of Transverse Waves

• The amplitude of any transverse wave is the distance from the crest or trough of the wave to the rest position of the medium. (AKA Resting point)

Behavior of waves

• 1. Reflection• 2. Refraction• 3. Diffraction• 4. Interference– Constructive Interference– Destructive Interference

• 5. Absorption

1. Reflection

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

• All types of wavesincluding sound, water, and light wavescan be reflected.

• EX: Echo, sonar (sound waves to detect an object)

The Law of Reflection

• The beam striking the 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.

The Law of Reflection

• The angle formed by the incident beam and the normal is the angle of incidence.

• The angle formed by the reflected beam and the normal is the angle of reflection.

The Law of Reflection

• According to the law of reflection, the angle of incidence is equal to the angle of refection. All reflected waves obey this law.

2. Refraction

• When a wave passes from one medium to anothersuch as when a light wave passes from air to waterit changes speed.

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

• 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.

• When light waves travel from air to water, they slow down and bend toward the normal.

Refraction of Light in Water

• When light waves travel from water to air, they speed up and bend away from the normal.

Refraction of Light in Water

3. Diffraction

• 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.

Diffraction

• Waves also can be diffracted when they pass through a narrow opening.

• After they pass through the opening, the waves spread out.

• The amount of diffraction that occurs depends on how big the obstacle or opening is compared to the wavelength.

• Allows you to hear in hallways

4. Interference

• When two or more waves overlap and combine to form a new wave, the process is called interference. Interference occurs while two waves are overlapping.

• There are 2 types!

4 a. Constructive Interference • In constructive interference, the waves

add together.• This happens when

the crests of two or more transverse waves arrive at the same place at the same time and overlap.

• Amplitude is the sum of the two waves

4b. Destructive Interference

• In destructive interference, the waves subtract from each other as they overlap.

• This happens when the crests of one transverse wave meet the troughs of another transverse wave.

• Amplitude is the difference between the two waves

5. Absorption

• The wave can be absorbed and disappear.

Standing Waves

• A standing wave is 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.

Resonance

• The process by which an object is made to vibrate by absorbing energy at its natural frequencies is call resonance.

• If enough energy is absorbed, the object can vibrate so strongly that it breaks apart.

• EX: Tacoma Narrows Bridge

Intensity

Intensity is the amount of energy that flows through a certain area in a specific amount of time.

IntensityWhen you turn down the volume of your radio,

you reduce the energy carried by the sound waves,

so you also reduce their intensity.

Loudness

Loudness is the human perception of sound intensity.

When sound waves of high intensity reach your ear, they cause your eardrum to move back and forth a greater distance than sound waves of low intensity do.

A Scale for Loudness

The intensity of sound can be described using a measurement scale.

Each unit on the scale for sound intensity is called a

decibel (DE suh bel), abbreviated dB.

Ultrasonic and Infrasonic Waves

Most people can’t hear sound frequencies above 20,000 Hz, which are called ultrasonic waves.

Even though humans can’t hear ultrasonic waves, they use them for many things.

Ultrasonic and Infrasonic Waves

They also are used to estimate the size, shape, and depth of underwater objects.

Ultrasonic waves are used in medical diagnosis and treatment.

These waves are produced by sources that vibrate slowly, such as wind, heavy machinery, and earthquakes.

Infrasonic Waves

The Doppler Effect

The change in pitch or wave frequency due to a moving wave source is called the Doppler effect.

The Doppler effect occurs when the source of a sound wave is moving relative to a listener.

Moving Sound

As a race car moves, it sends out sound waves in the form of compressions and rarefactions.

The race car creates a compression, labeled A.

Compression A moves through the air toward the flagger standing at the finish line.

Moving Sound

By the time compression B leaves the race car, the car has moved forward.

Because the car has moved since the time it created compression A, compressions A and B are closer together than they would be if the car had stayed still.

Moving Sound

As a result, the flagger hears a higher pitch.

A Moving Observer

The Doppler effect happens any time the source of a sound is changing position compared with the observer.

It occurs no matter whether it is the sound source or the observer that is moving.

The faster the change in position, the greater the change in frequency and pitch.

• Electromagnetic waves are made by vibrating electric charges and can travel through space where matter is not present.

• Instead of transferring energy from particle to particle, electromagnetic waves travel by transferring energy between vibrating electric and magnetic fields.

Electromagnetic Waves

• Just as magnets are surrounded by magnetic fields, electric charges are surrounded by electric fields.

• An electric field enables charges to exert forces on each other even when they are far apart.

Electric and Magnetic Fields

• An electric field exists around an electric charge even if the space around it contains no matter.

• Electric charges also can be surrounded by magnetic fields.

• An electric current flowing through a wire is surrounded by a magnetic field, as shown.

Magnetic Fields and Moving Charges

• An electric current in a wire is the flow of electrons in a single direction.

• It is the motion of these electrons that creates the magnetic field around the wire.

Magnetic Fields and Moving Charges

• A changing magnetic field creates a changing electric field.

• The reverse is also truea changing electric field creates a changing magnetic field.

Changing Electric and Magnetic Fields

• Electromagnetic waves are produced when something vibratesan electric charge that moves back and forth.

• When an electric charge vibrates, the electric field around it changes.

Making Electromagnetic Waves

• Because the electric charge is in motion, it also has a magnetic field around it.

• A vibrating electric charge creates an electromagnetic wave that travels outward in all directions from the charge.

• The wave in only one direction is shown here.

Making Electromagnetic Waves

Making Electromagnetic Waves• Because the electric and magnetic fields

vibrate at right angles to the direction the wave travels, an electromagnetic wave is a transverse wave.

• As an electromagnetic wave moves, its electric and magnetic fields encounter objects.

• These vibrating fields can exert forces on charged particles and magnetic materials, causing them to move.

Properties of Electromagnetic Waves

• For example, electromagnetic waves from the Sun cause electrons in your skin to vibrate and gain energy, as shown.

• The energy carried by an electromagnetic wave is called radiant energy.

Properties of Electromagnetic Waves

• All electromagnetic waves travel at 300,000 km/s in the vacuum of space.

• The speed of electromagnetic waves in space is usually called the “speed of light.”

Wave Speed

• Because the energy carried by a wave depends on its amplitude and not its frequency, this result was mysterious.

Waves and Particles

• Years later, Albert Einstein provided an explanationelectromagnetic waves can behave as a particle, called a photon, whose energy depends on the frequency of the waves.

Properties of Light

• Electrons = the negatively charged particle in the atom involved in light. Electrons can absorb energy & emit energy in the form of light when they moved higher to lower energy levels.

A Range of Frequencies

• Electromagnetic waves can have a wide variety of frequencies.

• The entire range of electromagnetic wave frequencies is known as the electromagnetic spectrum.

THE ELECTROMAGNETIC SPECTRUM

• Parts of the electromagnetic spectrum

– Radio waves Longest ג Lowest Frequency– Microwaves– Infrared– Visible Light• ROY G B I V

– Ultraviolet– X-rays– Gamma rays Short ג Highest Frequency

• Radiowaves (1 km to 30 cm)– ex: AM & FM radio, TV signals, Radar detectors, MRI’s

• Microwaves (30 cm to 1 mm)– ex: microwave ovens (cooking), cellular communication

• Infrared (1 mm to 700 nm)– ex: heat (thermal energy), sensors, guidance systems

• Visible Light (70 nm to 400 nm)– ex: ROY G BIV

• Red Longest wavelength lowest frequency• Orange• Yellow• Green• Blue• Indigo• Violet Shortest wavelength highest frequency

– ex: Lasers = a single frequency of light– Optics = the study of light

• Ultraviolet– ex: sunburns, harmful - eyes, disinfect - killing

bacteria, fluorescence used in crime scenes

• X-Rays– ex: medical & dental uses, inspect metal welding

• Gamma Rays– ex: originate from – sun, atomic radiation

Opaque, Transparent, and Translucent

• Objects can absorb light, reflect light, and transmit lightallow light to pass through them.

• The type of matter in an object determines the amount of light it absorbs, reflects, and transmits.

Opaque, Transparent, and Translucent

• Opaque (oh PAYK) material only absorbs and reflects lightno light passes through it.

Opaque, Transparent, and Translucent

• Materials that allow some light to pass through them are described as translucent (trans LEW sunt).

• You cannot see clearly through translucent materials.

Opaque, Transparent, and Translucent

• Transparent materials transmit almost all the light striking them, so you can see objects clearly through them.

• Only a small amount of light is absorbed and reflected.