Waves

Waves

• The Nature of Waves–What’s in a Wave–Waves and

Energy–Mechanical

Waves

What is a Wave?

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

• Important: Waves transfer energy. The evidence is that they can do mechanical work (or destruction) or cause things to become warmer (transferring energy as heat).

• Example: 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.

What’s in a Wave?• Have you ever "done the wave" as part of a large

crowd at a football or baseball game?• A group of people jumps up and sits back down, some

nearby people see them and they jump up, some people further away follow suit and pretty soon you have a wave travelling around the stadium.

• The wave is the disturbance (people jumping up and sitting back down), and it travels around the stadium. However, none of the individual people in the stadium are carried around with the wave as it travels - they all remain at their seats.

Waves and Energy

• Suppose a pebble falls into a pool of water and ripples form.

• Because it is moving, the falling pebble has energy.

• As it splashes into the pool, the pebble transfers some of its energy to nearby water molecules, causing them to move.

• What you see is energy traveling in the form of a wave on the surface of the water.

Waves and Matter

• Imagine you’re in a boat on a lake.

• Approaching waves bump against your boat, but they don’t carry it along with them as they pass.

• The waves don’t even carry the water along with them. Only the energy carried by the waves moves forward.

• All waves have this property - they carry energy without transporting matter from place to place.

Making Waves

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

• 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 (2)

• 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

• Most waves travel through a medium.• A medium is the matter through which a

wave travels.• Sound waves travel through air to reach

your ears. Ocean waves travel through water to reach the shore. In both cases, the matter the wave traveled through is the medium.

• The 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.

Mechanical Waves (2)

• Waves that require a medium are called mechanical waves.

• Mechanical waves can only travel through matter.

• There are two types of mechanical waves: transverse waves and compressional waves.

• Light does not require a medium. Light waves are also called electromagnetic waves.

• An electromagnetic wave is a wave caused by a disturbance in electric and magnetic fields and does not require a medium.

Transverse Waves

• In a transverse wave, 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.

Transverse Waves (2)

The animation below shows a one-dimensional transverse plane wave passing from left to right. The particles do not move along with the wave; they simply vibrate up and down about their individual positions as the wave passes by.

Pick a single particle and watch its motion.

Compressional Waves • Compressional

waves are also called longitudinal waves.

• In a compressional wave, matter in the medium moves back and forth along the same direction that the wave travels.

• You can model compressional waves with a coiled spring toy.

Compressional Waves (2)

The animation below shows a one-dimensional compressional wave moving down a tube. The particles do not move down the tube with the wave; they simply vibrate back and forth about their individual positions.

Pick a single particle and watch its motion. The wave is seen as the motion of the compressed region which moves from left to right.

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.

• The air molecules are squeezed together like the coils in a coiled spring toy are when you make a compressional wave with it.

• The compressions travel through the air to make a wave.

Sound in Other Materials

• Sound waves also can travel through other mediums, such as water and wood.

• 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.

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 is pushed back and forth to form the crests and troughs.

Water Waves (2)

• The low point of a water wave is formed when water is pushed aside and up to the high point of the wave.

• The water that is pushed aside returns to its initial position.

Water Waves (3)The wave below is simulation of a water wave.

As a wave travels through the water, the particles travel in clockwise circles.

The simulation below shows a water wave travelling from left to right.

The two particles in blue show that each particle indeed travels in a clockwise circle as the wave passes.

Water Waves (4)

• 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 (2)

• 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.

The Parts of a Wave

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

Parts of a Wave (2)

• A compressional wave has no crests and troughs.

• When you make compressional waves in a coiled spring, a compression is a region where the coils are close together.

• The coils in the region next to a compression are spread apart, or less dense. This less-dense region of a compressional wave is called a 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 (2)

• 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).

• 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.

Wavelength is Related to Frequency (2)

As frequency increases, wavelength decreases.

Wave Speed

• Wave speed is simply how fast a wave moves.

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

Wave Speed Depends on Medium

• 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.

• The arrangement of particles in a medium determines how well waves travel through it. (Remember how particles are arranged in solids, liquids, and gases).

Amplitude and Energy

• The greatest distance that particles are displaced from their normal resting positions because of a wave is called the amplitude.

• 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.

Amplitude of Compressional 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 carries.

Amplitude of Compressional Waves (2)

• 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.

The Behavior of Waves

• Reflection• Refraction• Diffraction• Interference• Standing Waves

Reflection

• Reflection is the bouncing back of a wave when it meets a surface or boundary.

• The wave that reflects is like the original wave but moving in a new direction.

• All types of waves -including sound, water, and light waves - can be reflected.

Reflection (2)

• How does the reflection of light allow you to see yourself in the mirror? It happens in two steps. First, light strikes your face and bounces off. Then, the light reflected off your face strikes the mirror and is reflected into your eyes.

Echoes

• A similar thing happens to sound waves when your footsteps echo.

• Sound waves form when your foot hits the floor and the waves travel through the air to both your ears and other objects.

• Sometimes when the sound waves hit another object, they reflect off it and come back to you.

• Your ears hear the sound again, a few seconds after you first heard your footstep.

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.

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

Refraction

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

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

Refraction of Light in Water

• 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 (2)

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

Refraction of Light in Water (3)

• You may have noticed that objects that are underwater seem closer to the surface than they really are.

• In the figure, the light waves reflected from the swimmer’s foot are refracted away from the normal and enter your eyes.

• However, your brain assumes that all light waves have traveled in a straight line.

• The light waves that enter your eyes seem to have come from a foot that was higher in the water.

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 (2)

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

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

Diffraction and Wavelength

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

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

• If the obstacle is larger than the wavelength, the waves do not diffract as much. In fact, if the obstacle is much larger than the wavelength, almost no diffraction occurs.

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.

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.

• The amplitude of the new wave that forms is equal to the sum of the amplitudes of the original waves.

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.

• The amplitude of the new wave is the difference between the amplitudes of the waves that overlapped.

• Waves undergoing destructive interference are said to be out of phase.

Constructive and Destructive Interference

The sum wave is the blue wave traveling from left to right.When the two gray waves are in phase the result is larger amplitude (constructive interference).When the two gray waves become out of phase the sum wave is zero (destructive interference).

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.

Sound

Sound

What causes sound?

Sound Waves Speed of Sound Frequency and

Pitch The Doppler Effect

What Causes Sound?

• Every sound is produced by an object that vibrates.

• For example, your friends’ voices are produced by the vibrations of their vocal cords, and music from a carousel and voices from a loudspeaker are produced by vibrating speakers.

Sound Waves

Sound waves are compressional waves.

• A compressional wave is made up of two types of regions called compressions and rarefactions.

• You’ll see that when a radio speaker vibrates outward, the nearby molecules in the air are pushed together to form compressions.

Sound Waves (2)

• As the figure shows, when the speaker moves inward, the nearby molecules in the air have room to spread out, and a rarefaction forms.

• As long as the speaker continues to vibrate back and forth, compressions and rarefactions are formed.

Sound Waves (3)

• Compressions and rarefactions move away from the speaker as molecules in the air collide with their neighbors.

• A series of compressions and rarefactions forms that travels from the speaker to your ear.

• This sound wave is what you hear.

Moving through Materials

• Most sounds you hear travel through air to reach your ears.

• If you’ve ever been swimming underwater and heard garbled voices, you know that sound also travels through water.

• Sound waves can travel through any type of mattersolid, liquid, or gas.

• Remember the matter that a wave travels through is called a medium.

• Sound waves cannot travel through empty space.

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.

• In general, sound travels the slowest through gases, faster through liquids, and even faster through solids.

The Speed of Sound in Different Materials (2)

• Sound travels faster in liquids and solids than in gases because the individual molecules in a liquid or solid are closer together than the molecules in a gas.

• However, the speed of sound doesn’t depend on the loudness of the sound.

• Loud sounds travel through a medium at the same speed as soft sounds.

Pitch

If you were to sing a scale, your voice would start low and become higher with each note.

Pitch is how high or low a sound seems to be.

The pitch of a sound is related to the frequency of the sound waves.

Frequency and Pitch

Frequency is a measure of how many wavelengths pass a particular point each second.

For a compressional wave, such as sound, the frequency is the number of compressions or the number of rarefactions that pass by each second.

Frequency is measured in hertz (Hz).

Frequency and Pitch (2)

When a sound wave with high frequency hits your ear, many compressions hit your eardrum each second.

Your brain interprets these fast vibrations caused by high-frequency waves as a sound with a high pitch.

As the frequency of a sound wave decreases, the pitch becomes lower.

Frequency and Pitch (3)

This figure shows different notes and their frequencies.

A healthy human ear can hear sound waves with frequencies from about 20 Hz to 20,000 Hz.

The human ear is most sensitive to sounds in the range of 440 Hz to about 7,000 Hz.

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.

The Doppler Effect (2)

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.

The Doppler Effect (3)

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.

The Doppler Effect (4)

As a result, the flagger hears a higher pitch. 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.

Using the Doppler Effect

The Doppler effect also occurs for other waves besides sound waves.

For example, the frequency of electromagnetic waves, such as radar waves, changes if an observer and wave source are moving relative to each other.

Using the Doppler Effect (2)

Radar guns use the Doppler effect to measure the speed of cars.

Weather radar also uses the Doppler shift to show the movement of winds in storms, such as a tornado.

Light

Light

Light and Matter

Prisms Colors Lenses

Light and Matter

What you see depends on the amount of light in the room and the color of the objects.

For you to see an object, it must reflect some light back to your eyes.

Remember reflection occurs when a light wave strikes an object and bounces off.

Objects can absorb light, reflect light, and transmit light (allow light to pass through them).

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

Light and Matter

Opaque material only absorbs and reflects light (no light passes through it).

As a result, you cannot see the candle inside.

Light and Matter

Materials that allow some light to pass through them are described as translucent.

You cannot see clearly through translucent materials.

Light and Matter

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.

Prisms Refraction causes a

prism to separate a beam of white light into different colors.

Remember refraction is caused by a change in the speed of a wave as it passes from one material to another.

How does the bending of light create these colors?

It occurs because the amount of bending usually depends on the wavelength of the light.

Prisms

Wavelengths of visible light range from the longer red waves to the shorter violet waves.

White light, such as sunlight, is made up of this whole range of wavelengths.

The animation shows what happens when white light passes through a prism.

Prisms The triangular prism refracts

the light twice – once when it enters the prism and again when it leaves the prism and reenters the air.

Because the longer wavelengths of light are refracted less than the shorter wavelengths are, red light is bent the least.

As a result of these different amounts of bending, the different colors are separated when they emerge from the prism.

Which color of light would you expect to bend the most?

Rainbows

Does the light leaving the prism remind you of a rainbow?

Like prisms, rain droplets also refract light.

The refraction of the different wavelengths can cause white light from the Sun to separate into the individual colors of visible light.

Rainbows

In a rainbow, the human eye usually can distinguish only about seven colors clearly.

In order of decreasing wavelength, these colors are red, orange, yellow, green, blue, indigo, and violet.

Colors

Why do some apples appear red, while others look green or yellow?

An object’s color depends on the wavelengths of light it reflects.

Remember white light is a blend of all colors of visible light.

Colors

When a red apple is struck by white light, it reflects red light back to your eyes and absorbs all of the other colors.

The figure shows white light striking a green leaf. Only the green light is reflected back to your eyes.

Colors

Although some objects appear to be black, black isn’t a color that is present in visible light.

Objects that appear black absorb all colors of light and reflect little or no light back to your eye.

White objects appear to be white because they reflect all colors of visible light.

Lenses

What do your eyes have in common with cameras, eyeglasses, and microscopes?

Each of these things contains at least one lens.

A lens is a transparent material with at least one curved surface that causes light rays to bend, or refract, as they pass through.

The image that a lens forms depends on the shape of the lens.

Lenses can be convex or concave.

Convex Lenses Convex lenses are

thicker in the middle than at the edges.

A convex lens focuses light at a focal point.

The focal length of the lens depends on the shape of the lens.

If the sides are less curved, light rays are bent less.

As a result, lenses with flatter sides have longer focal lengths.

Forming Images with a Convex Lens

The type of image a convex lens forms depends on where the object is relative to the focal point of the lens.

When the candle is more than two focal lengths away from the lens, its image is real, reduced, and upside down.

A real image is an image formed by light rays that converge to pass through the place where the image is located.

Forming Images with a Convex Lens

When the candle is between one and two focal lengths from the lens, its image is real, enlarged, and upside down.

Forming Images with a Convex Lens

When the candle is less than one focal length from the lens, its image is virtual, enlarged, and upright.

A virtual image is an image formed by diverging light rays that is perceived by the brain, even though no actual light rays pass through the place where the image seems to be located.

Concave Lenses A concave lens is

thinner in the middle and thicker on the edges.

Light rays that pass through a concave lens bend outward away from the optical axis.

The image formed is always virtual, upright, and smaller than the actual object is.

Concave lenses are used in some types of eyeglasses and some telescopes.

Lenses and Eyesight Light enters your eye through a

transparent covering on your eyeball called the cornea.

The cornea causes light rays to bend so that they converge.

The light then passes through an opening called the pupil.

Behind the pupil is a flexible convex lens.

The lens helps focus light rays so that a sharp image is formed on your retina.

The retina is the inner lining of your eye.

It has cells that convert the light image into electrical signals, which are then carried along the optic nerve to your brain to be interpreted.