WAVES Types of Waves and Their Properties Questions

WAVES Types of Waves and Their Properties

Questions of the Week �What is a wave? �What kinds of waves are there? http: //brightstorm. com/science/p hysics/vibration-andwaves/wave-characteristics/

Waves Are Everywhere!!! � Waves are disturbances that transfer energy through matter or space. � Energy is the ability to do work. � There are 2 main types of waves: � 1) Waves that require a medium to travel through � Waves that need a medium are called mechanical waves. � A medium is the material (matter) in which a mechanical wave travels. � A medium can be a solid, liquid, gas, or a combination. � All waves that travel through a medium have a source. � 2) Waves that are able to travel without a medium � � eg. electromagnetic (light) waves can travel through a vacuum (empty space where there is no matter), but can also travel through a medium We will learn more about EM waves when we talk about light later in the Unit. � Khan Academy – Introduction to Waves

Mechanical Waves � Mechanical waves are created when a source of energy causes a medium to vibrate or oscillate. � Vibrations are repeated up-and- down or back-and-forth movement of molecules or objects � Sound and water waves travel through a medium. � The medium requires matter � A medium can be a solid, liquid, gas, or a combination of these. � There are 3 types of mechanical waves, based on the direction the wave moves: � longitudinal, transverse, and surface �Vibrations Video #1

Transverse Waves � A transverse wave is when matter in the medium moves at right angles (perpendicular, 90°, or up and down) to the direction the wave travels. � eg. a rope, electromagnetic radiation (light), seismic waves (earthquakes) � The crest is the highest part of the wave. � The trough is the lowest point of the wave. � Rest is where the wave would be if it wasn´t moving (in the middle of the crest and the trough). � Transverse wave video � Brightstorm - Transverse Waves

Longitudinal Waves � A longitudinal wave (compression wave) is when matter in a medium moves in the same direction (parallel or back and forth) that the wave travels. � eg. slinky; sound � A compression is when the medium is pushed close together (or is more dense). � A rarefaction is when the medium is pulled farther apart (or is less dense). �Longitudinal wave video �Transverse vs. Longitudinal wave �Brightstorm – Longitudinal Waves

Types of Mechanical Waves

Surface Waves �A surface wave is a combination of the two types of waves (longitudinal and transverse). � The particles move in a circular motion. �It occurs between two mediums; for example air and water. �Water moves back and forth slightly but can not compress �Brain. Pop - Waves

Question of the Week �If waves are similar to one another, how can we tell them apart?

Properties of Waves � There are many different types of waves (eg. sound, light, etc. ), but all waves have the same basic shape � Waves share certain properties � What distinguishes one wave from another are: � The amplitude (height) of the wave � The wavelength (distance) of the wave � The frequency (number of waves in a certain time) � The speed (how fast) at which a wave travels � Khan Academy – Amplitude, Frequency, and Wavelength of Waves

Which Wave Has More Energy? �When you go surfing, do you try to catch: �A little wave… �… Or a BIG wave?

Amplitude �The amplitude of a wave is the height of the crest or the depth of the trough (they are the same). à In a transverse wave, amplitude is the distance from rest to crest (or rest to trough)… but not crest to trough! à In a longitudinal wave, amplitude is how compressed a wave is. � The larger the amplitude, the greater the energy a wave has. � The smaller the amplitude, the less energy a wave has. Which wave has the highest amplitude? � Amplitude video � Wave Characteristics video

Wavelength �The wavelength (λ) is the distance between two corresponding parts of the wave � eg. in a transverse wave, it is measured from crest to crest, or from trough to trough � eg. in a longitudinal wave, it is measured from compression to compression or from rarefaction to rarefaction � Wavelength is measured in meters (m) �Wavelength video

Frequency � The frequency (f) of a wave is the number of times the wave passes a certain point in a given time � It is measured in vibrations/sec or Hertz (Hz) � 1 Hz = 1 vibration/sec or 1 wave/sec � eg. If a string vibrates three times in 10 seconds, what is the frequency? � � � 3 vibrations/10 seconds = 0. 3 vibrations/sec = 0. 3 Hz Which wave has the highest frequency? Why? � Brightstorm - Frequency

Frequency Questions 1. Use the two waves on the right to answer the following questions: � a) What are the frequencies of these waves if they pass by in 10 seconds? � b) What are their frequencies if they pass by in 1 second? 2. What happens to the frequency of a wave if we change the wavelength? à As the wavelength decreases, the frequency increases. à … and vice versa: as the wavelength increases, the frequency decreases. �Waves Song

Important Facts About Waves �Light travels about 300 000 m/s (300 000 km/s)! �Sound travels about 340 m/s (in air). � The speed of a wave depends on the properties of the medium the wave is traveling in. � eg. Sound travels faster in solids or liquids than in a gas �As long as waves travel through the same medium, their speed is unaffected by a change in wavelength or frequency because as one increases, the other decreases �The higher the amplitude of a wave, the larger its energy �The longer the wavelength, the lower the frequency and the lower the pitch (pitch is highness or lowness of sounds)

Question of the Day �How can we calculate the speed, frequency, or wavelength of a wave? �Brightstorm - Wave Speed

Wave Calculations � When talking about Energy, we learned that velocity is how fast something is moving. � velocity = distance ÷ time � units: m/s = m ÷ s (or v = d/t) � When we talk about waves: � the distance is wavelength of the wave (in meters); and � the time is the inverse of the wave´s frequency (in 1 ÷ s or 1/s) [another name for this is the period (T)] � So the equation to find the speed of a wave becomes: � speed = wavelength x frequency … or in other words: � When we rearrange the formulas to solve for λ x f: v ÷ λ v=λxf ÷ x f � eg. A wave has a wavelength of 1 m. If it has a frequency of 10 Hz, how quickly is it traveling? � � knowns: v=λxf v = 1 m x 10 Hz v = 10 m/s f = 10 Hz; λ = 1 m unknown: v

Wave Calculations �A sound wave is traveling at 340 m/s. If the frequency is 68 Hz, what is the wavelength? �If a microwave has a wavelength of 5 mm and travels 300, 000 km/s, what is its frequency?

Wave Calculations � A sound wave is traveling at 340 m/s. If the frequency is 68 Hz, what is the wavelength? � knowns: f = 68 Hz; v = 340 m/s unknown: λ �v = λ x f � λ = v/f � λ = 340 m/s ÷ 68 Hz �λ = 5 m � If a microwave has a wavelength of 5 mm and travels 300, 000 km/s, what is its frequency? � knowns: λ = 5 mm, v = 300, 000 km/s unknown: f � Must convert to SI units (eg. meters) �v = λ x f f = v/ λ � f = 300, 000 m/s ÷ 0. 005 m � f = 60, 000, 000 /s (or Hz) λ = 0. 005 m, v = 300, 000 m/s

Practice Wave Calculations � 1. A 100 waves pass by every 20 seconds. What is the wave’s frequency? � 2. Calculate the velocity of a wave if its wavelength is 30 cm and its frequency is 150 Hz. � 3. Find the wavelength of a wave if it travels at 10 m/s and has a frequency of 8000 Hz. � 4. What is the frequency of a light wave that has a wavelength of 30 mm? � 5. A wave travels 13 km in 6. 5 seconds. What is its velocity?

Practice Wave Calculations � 1. A 100 waves pass by every 20 seconds. What is the wave’s frequency? � 0. 2 Hz � 2. Calculate the velocity of a wave if its wavelength is 30 cm and its frequency is 150 Hz. � 45 m/s � 3. Find the wavelength of a wave if it travels at 10 m/s and has a frequency of 8000 Hz. � 0. 00125 m � 4. What is the frequency of a light wave that has a wavelength of 30 mm? � 1 x 1011 Hz (or 100 000 000 Hz) � 5. A wave travels 13 km in 6. 5 seconds. What is its velocity? � 2000 m/s

Practice Wave Calculations �Mac and Tosh are resting on top of the water near the end of the pool when Mac creates a surface wave. The wave travels the length of the pool and back in 25 seconds. The pool is 25 meters long. Determine the speed of the wave. �While hiking through a canyon, Noah Formula lets out a scream. An echo (reflection of the scream off a nearby canyon wall) is heard 0. 82 seconds after the scream. The speed of the sound wave in air is 342 m/s. Calculate the distance from Noah to the nearby canyon wall.

Question of the Day �What happens to a wave when it encounters an obstacle or another wave?

Wave Behaviour and Other Phenomena �When a wave travels through a medium, it will often encounter the end of the medium or some other obstacle. �This obstacle affects the way that the wave behaves.

Reflection �Reflection occurs when a wave hits a reflective surface and the wave changes direction and bounces back. �Law of reflection - the angle of incidence = the angle of reflection. �eg. Ball hits wall, look in mirror, echo

Refraction �Refraction is the change in direction of a wave due to a change in its speed. � This happens when a wave goes from one medium to another �When a wave moves from one medium to another at an angle, it changes speed and bends. �NOT the same thing as rarefaction in a longitudinal wave

Diffraction �Diffraction is the bending of waves around the edge of a barrier. � eg. sound bends around corners �Because of change of speed, waves can bend in different patterns � The amount of bending increases with increasing wavelength and decreases with decreasing wavelength � Brain. Pop – Refraction vs. Diffraction

Interference � Interference occurs when two or more waves meet and interact with each other � Constructive interference when two waves combine to make a wave with a larger amplitude. � Constructive Interference Video � Destructive interference -when two waves meet and make a wave with a smaller amplitude or cancel each other out. � Destructive Interference Video

Standing Wave � A standing wave is a wave that appears to stand in one place (constant position) � This happens when two waves traveling in opposite direction interfere with one another � A node is a point where the amplitude of the wave is zero � An antinode is the maximum energy in the wave (the crests and troughs) � Standing Wave Video #2

Resonance � Most objects have a natural frequency of vibration. � Resonance is when vibrations traveling through an object match the object’s natural vibrations. � If vibrations of the same frequency are added, the amplitude increases � eg. a swing (if you time the pushes properly, the swing moves with a large amplitude) � eg. breaking a wine glass � Many of the sounds we hear, such as when hard objects are struck (metal, glass, wood, etc), are caused by brief resonant vibrations in that object. http: //www. brightstorm. com/science/physics/oscillatory-motion/resonance/ http: //www. youtube. com/watch? v=j-zcz. JXSxnw