WAVES

1. WAVES Types of Waves and Their Properties

2. Questions of the Week • What is a wave? • What kinds of waves are there? http://brightstorm.com/science/physics/vibration-and-waves/wave-characteristics/

3. 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 mechanicalwaves. • 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 nomatter), 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

4. 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

5. 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

6. 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

7. Types of Mechanical Waves

8. Surface Waves • A surfacewaveis a combination of the two types of waves (longitudinal and transverse). • The particles move in a circular motion. • It occurs between twomediums; for example air and water. • Water moves back and forth slightly but can not compress • BrainPop - Waves

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

10. 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

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

12. Amplitude • The amplitude of a wave is the height of the crest or the depth of the trough (they are the same). • In a transverse wave, amplitude is the distance from rest to crest (or rest to trough)… but not crest to trough! • 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. • Amplitude video • Wave Characteristics video Which wave has the highest amplitude?

13. 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

14. 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 • Brightstorm - Frequency Which wave has the highest frequency? Why?

15. 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? • As the wavelength decreases, the frequency increases. • … and vice versa: as the wavelength increases, the frequency decreases. • Waves Song

16. Important Facts About Waves • Light travels about 300 000 000 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)

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

18. Wave Calculations • When talking about Energy, we learned that velocity is how fast something is moving. • velocity = distance ÷ time (or v = d/t) • units: m/s = m÷s • 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: v = λxf • When we rearrange the formulas to solve for λxf: v ÷ ÷ λxf • eg. A wave has a wavelength of 1 m. If it has a frequency of 10 Hz, how quickly is it traveling? • knowns: f = 10 Hz; λ = 1 munknown: v • v = λ x f • v = 1 m x 10 Hz • v = 10 m/s

19. 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?

20. 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/sunknown: 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) λ = 0.005 m, v = 300,000,000 m/s • v = λ x f f = v/ λ • f = 300,000,000 m/s ÷ 0.005 m • f = 60,000,000,000 /s (or Hz)

21. 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?

22. 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 100 000 000 000 Hz) • 5. A wave travels 13 km in 6.5 seconds. What is its velocity? • 2000 m/s

23. 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.

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

25. 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.

26. 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

27. 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

28. 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 • BrainPop – Refraction vs. Diffraction

29. 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

30. 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 • Standing Wave Video #2

31. 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-zczJXSxnw