Waves Unit 8 Chapter 11

Topics to be covered in this unit 1. Types of Waves 2. Characteristics of Waves 3. Wave Interactions

What is a wave? Waves are disturbances that transmit energy through matter or space. –Example – when you throw a stone into a pond… it creates a ripple on the surface of the water.

How waves travel Ripples in the pond Sound traveling through air Earthquakes in the ground These are all examples of mediums in which waves can travel. A medium is the matter through which a wave travels.

How waves travel Waves that require a medium are called mechanical waves. Almost all waves require a medium –Exceptions are electromagnetic waves

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

Light Light can travel from the sun to the Earth through the empty space between the two. Light does not need a medium Light waves consist of changing electric and magnetic fields in space. They are also called electromagnetic waves –Examples: visible light and radio waves

Waves transfer energy Remember – energy is the ability to do work. Waves carry energy because they can do work. –Water waves can move a boat or leaf –Sound waves impact your eardrum –Light waves work on your eye The bigger the wave, the more energy it carries.

Tsunami

Waves transfer energy Tsumani waves carry enough energy to cause a lot of damage to coastal towns and shorelines. Normal-size waves do work on the shore to break up rocks into tiny pieces forming sandy beaches.

Energy may spread out as a wave travels The closer you are to speakers, the louder the music. Waves spread out in circles that get bigger as the waves move farther from the center. Each circle (called a wave front) has the same amount of total energy. But as the circle get larger, the energy spreads out over a larger area.

Vibrations and Waves Waves are related to vibrations –Example: a singer sings (vocal cords move back and forth) creating sound. The sound waves reach your ears causing your eardrum to vibrate. This vibration is interpreted by your brain as sounds. Most waves are caused by a vibrating object Electromagnetic waves may be caused by vibrating charged particles In a mechanical wave, the particles in the medium also vibrate as the wave passes through the medium.

Vibrations involve transformations of energy Vibration: in a general sense, anything that switches back and forth, to and from, side to side, in and out, off and on, loud and soft, or up and down is vibrating. A vibration is a wiggle in time. Wave: a wiggle in both space and time is a wave. A wave extends from one place to another.

Vibrations involve transformations of energy Vibrations and waves: the source of all waves is something that is vibrating. Waves are propagations of vibrations throughout space. Example is a pendulum in a clock.

simple harmonic motion is a type of motion where the restoring force is directly proportional to the displacement

Simple Harmonic Motion A wave whose source vibrates with simple harmonic motion is called a sine wave As the mass moves down, the spring is exerting force which pushes it back to the original position. This will allow the mass to keep bouncing up and down forever.

Damped Harmonic Motion Unlike the simple harmonic motion which will continue forever. The damped harmonic motion will transfer its energy to the second mass, it slows down and then returns to its resting position. u/jw/oscillations.htm#Dampedhttp:// u/jw/oscillations.htm#Damped

Wave Types Transverse waves vibrate across from direction of travel Longitudinal waves vibrate along the direction of travel (as in a spring)

20 Longitudinal Wave The wave we see here is a longitudinal wave. In a longitudinal wave, the medium particles vibrate parallel to the motion of the pulse. This is the same type of wave that we use to transfer sound.

21 Transverse waves A second type of wave is a transverse wave. We said in a longitudinal wave the pulse travels in a direction parallel to the disturbance. In a transverse wave the pulse travels perpendicular to the disturbance.

22 What’s the difference? The differences between the two can be seen

Surface Wave Surface waves occur at the boundary between two different mediums such as between water and air. Example: The waves on the ocean are not simply transverse or longitudinal waves.

Characteristics of Waves Waves can be observed with many different properties: –Large or small –Close together or very far apart However, they can all be described with their properties using a sine curve.

Wave Properties Wavelength Wavelength, , is the distance between any two successive identifical parts of a wave.

Wave Properties Amplitude Amplitude is the greatest distance that particles in a medium move from their normal position when a wave passes.

Parts of a Wave Crest: The highest point of a transverse wave

Parts of a Wave Trough: The lowest point of a transverse wave. These are the points where the wave exhibits its maximum negative or downward displacement.

Longitudinal wave parts: Compressions- a region in a longitudinal wave where the particles are closest together. It is an area of high density Rarefactions- A decrease in density and pressure in a medium.

Wave Properties Period The wave period, P, the time required for one full wavelength to pass a certain point.

Wave Properties Frequency Frequency, f, is the number of vibrations that occur in a 1 second time interval.

Frequency-Period Equation The frequency and period of a wave are related. –If more vibrations are made in a second, each one takes a shorter amount of time. –The frequency is inverse of the period. SI unit is Hertz (Hz) Frequency = 1 period f = 1/T

Visible Light Our eyes can detect light with frequencies from 4.3 x Hz to 7.5 x Hz. This is called the visible light

Electromagnetic Spectrum Light occurs at other ranges we cannot see with our eyes. The full range is called the electromagnetic spectrum.

Type of wave MechanicalElectromagnetic FormLongitudinalTransverse Modeled as transverse MediumSolids, liquids, gases Solids and liquids None required Travel asCompression and rarefactions in matter Back & forth movement of matter Oscillating electric and magnetic fields ExamplesSound waves and some earthquake waves Water waves, rope waves, some earthquake waves Visible light waves, radio waves, X-rays

Wave speed Wave speed is the speed at which a wave passes through a medium. Formula: Wave speed = frequency x wavelength v = f x SI unit for wave speed is meters per second. m/s

Practice problem The average wavelength in a series of ocean waves is 15.0 m. A wave arrives on average every 10.0 s, so the frequency is Hz. What is the average speed of the waves? v = f x

Practice problem The average wavelength in a series of ocean waves is 15.0 m. A wave arrives on average every 10.0 s, so the frequency is Hz. What is the average speed of the waves? v = f x = Hz x 15.0 m = 1.5 m/s

Practice problems v = f x P = 1 t

Sound Waves Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.

Doppler Effect Doppler effect is an observed change in the frequency of a wave when the source or observer is moving. Pitch of a sound, how high or low it is, is determined by the frequency at which sound waves strike the eardrum in your ear.

Of course….. There's this take on it

Doppler Effect Refers to the change in frequency when there is relative motion between an observer of waves and the source of the waves Doppler with Sound

Red shift?

Expand that….. Wrap your mind around this

Wave Interactions Two different material objects can never occupy the same space at the same time. When two waves come together they do not bounce back as bumper boats. Because mechanical waves are not matter but rather the displacement of matter, two waves can occupy the same space at the same time

Wave Interactions Reflection is the bouncing back of a wave as it meets a surface or boundary angle of incidence=angle of reflection

Reflection of a wave from a free end Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Reflection of a wave from a fixed end Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Wave Interactions Diffraction is the bending of a wave as it passes an edge or an opening

Wave Interactions Refraction is the bending of waves as the pass from one medium to another.

What is the difference? efractionanddiffraction/ efractionanddiffraction/

59 Wave Interaction All we have left to discover is how waves interact with each other. When two waves meet while traveling along the same medium it is called INTERFERENCE. Interference is the combination of two or more waves that exist in the same place at the same time.

Wave Interference When two wave pass each other their superposition causes reinforcement or cancellation.

Constructive interference Reinforcement when the crest of one wave overlaps the crest of another Their individual effects adds together, resulting in a wave increased in amplitude In phase- two waves crests and troughs arrive at a place at the same time, effects reinforce each other

Destructive Interference Cancellation when crest of one wave overlaps trough of another reducing their individual effects Water waves show these best Out of phase- the crest of one wave arrives at a point at the same time as a trough of the second wave arrives, effects cancel each other

Sound Wave Interference Interference occurs when two sounds of difference frequency are heard superposed. Constructive interference (compressions align) causes louder sound and destructive inference cause fainter sound This alternating pattern produces a beat.

Shock Waves

Water Wave Interference Left side is theoretical drawing of an interference pattern. Right side is the actual interference pattern.

Standing Waves Standing wave is a wave form caused by interference that appears not to move along the medium and that shows some regions of no vibrations (nodes)….

Standing Waves And other regions of maximum vibrations called antinodes. Antinodes form where the crests of the original waves line up with the crests of the reflected waves so that complete constructive interference occurs.

Standing Waves Change the frequency in a standing wave and more nodes/antinodes appear in the event

Review Questions

What dictates the frequency of a sound wave? (a) wavelength (b) medium (c) source of the sound (d) speed (e) amplitude

What dictates the frequency of a sound wave? (a) wavelength (b) medium (c) source of the sound (d) speed (e) amplitude

What determines the speed of a wave? (a) the frequency (b) the wavelength (c) the amplitude (d) the period (e) the medium of transmission

What determines the speed of a wave? (a) the frequency (b) the wavelength (c) the amplitude (d) the period (e) the medium of transmission

A skipper on a boat notices wave crests passing his anchor chain every 5 seconds. If the wave crests are 15 m apart, what is the speed of the water waves in m/s? (a) 5 (d) 10 (b) 15 (e) 3 (c) 75

A skipper on a boat notices wave crests passing his anchor chain every 5 seconds. If the wave crests are 15 m apart, what is the speed of the water waves in m/s? (a) 5 (d) 10 (b) 15 (e) 3 (c) 75

For a medium transmitting a longitudinal wave, the areas of the medium where the density of the medium is temporarily increased are called... (a) rarefactions (b) compressions (c) density holes

For a medium transmitting a longitudinal wave, the areas of the medium where the density of the medium is temporarily increased are called... (a) rarefactions (b) compressions (c) density holes

When you move away from a fixed source of sound, the frequency of the sound you hear... (a) is greater than what the source emits (b) is less than what the source emits (c) is the same as what the source emits

When you move away from a fixed source of sound, the frequency of the sound you hear... (a) is greater than what the source emits (b) is less than what the source emits (c) is the same as what the source emits

Sonic booms from a plane are produced... (a) because the plane breaks through the sound barrier. (b) when the plane reaches the speed of sound. (c) by the plane traveling faster than the speed of sound. (d) by the plane traveling slower than the speed of sound.

Sonic booms from a plane are produced... (a) because the plane breaks through the sound barrier. (b) when the plane reaches the speed of sound. (c) by the plane traveling faster than the speed of sound. (d) by the plane traveling slower than the speed of sound.

Type of wave MechanicalElectromagnetic FormLongitudinalTransverse Modeled as transverse MediumSolids, liquids, gases Solids and liquids None required Travel asCompression and rarefactions in matter Back & forth movement of matter Oscillating electric and magnetic fields ExamplesSound waves and some earthquake waves Water waves, rope waves, some earthquake waves Visible light waves, radio waves, X-rays