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Waves, Sound and Light

Waves, Sound and Light. Wave Properties. What is a wave?. Disturbance or vibration that transmits energy but not matter Examples… Sound, light, radiowaves , earthquakes. Types of Waves. Mechanical Waves- Use a medium Transverse waves- particles move perpendicular to wave motion

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Waves, Sound and Light

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  1. Waves, Sound and Light Wave Properties

  2. What is a wave? • Disturbance or vibration that transmits energy but not matter • Examples… • Sound, light, radiowaves, earthquakes

  3. Types of Waves • Mechanical Waves- Use a medium • Transverse waves- particles move perpendicular to wave motion • Longitudinal Waves- particles move parallel to wave motion

  4. Parts Of A Wave • Crest-top of the wave • Trough- Bottom of the wave • Amplitude (A)-height from resting position • Wavelength (λ)- distance travelled by a single wave

  5. Frequency and Period • Period (T)- time for one complete cycle or wavelength (in s) • Frequency (f)- number of waves per second (Hertz, Hz) • Hertz=1/s or s-1 • Frequency and period are reciprocals of themselves • f= 1/T • T= 1/f • They are inversely • proportional

  6. Example • Playing middle C on a piano produces a sound with a frequency of 256 Hz. What is the period of the sound wave? • f= 256 Hz • T=? • T=1/f • T=1/256 • T=0.004 s

  7. Speed of Waves • Remember • V=d/t • So to get speed we need distance and time • A single wave • Distance travelled is one wavelength, λ • Time is one period, T • Velocity of a wave=wavelength x frequency • v=λf

  8. Example • An air horn sound at a frequency of 220 Hz. If the speed of sound in air is 330 m/s, what is the wavelength of the sound wave?

  9. Example • The distance between successive crest in a series of water is 4.0 m, and the crests travel 8.6 m in 5.0 seconds. Calculate the frequency of a block of wood bobbing up and down on these water waves. • λ= 4.0 m • d=8.6 m • t=5.0 s • f=? • V=λf • V=d/t • V= 8.6/5 • V=1.72 m/s • 1.72=4f

  10. Waves Interference

  11. Interference • Two different material objects can not occupy the same space at the same time • Waves are not matter but they can displace matter • Waves can occupy the same place at the same time • When waves overlap it is called superposition

  12. Constructive Interference • Two waves superimposing that are on the same side of equilibrium • They will enhance each other or add up

  13. Destructive Interference • Waves are on opposite sides of equilibrium • They will weaken each other or subtract

  14. Standing Waves • A wave that remains in constant position • Can occur when a medium is moving in the opposite direction or when two waves interfere with each other, or constructive and destructive interference

  15. Reflection From a Boundary • When waves hit a boundary the wave will be reflected • If the boundary is fixed the wave will be reflected AND INVERTED • If the boundary is free the wave will just be reflected

  16. Standing Waves • If a series of waves are sent along a string the reflected pulse will interfere with itself • If the waves are sent at just the right frequency we will create a standing wave • Maximum Wavelength on a standing wave is 2L

  17. Antinodes and Nodes • Areas of complete destructive interference have NO amplitude are nodes • Areas of complete constructive interference have LARGE amplitudes are antinodes

  18. Waves Sound

  19. Sound • Begins with a vibrating object • Is a longitudinal wave • The compression is where density and pressure are at a maximum (crest) • The rarefaction is the region are the minimum points (trough)

  20. Sound Characteristics • Needs a medium to travel • The higher the temperature the faster • Higher temp=more energy=more vibrations • The more dense the faster • Vibrations will occur more quickly if the molecules are closer together • Pitch is determined by frequency • Volume is determined by amplitude

  21. Clicker Question • Which statement is true? • A) If a tree fell in outer space no one can hear it • B) A dog whistle has produces sounds that have a high amplitude • C) If you are at a concert all of the sound waves have a high frequency • D) Sound travels faster in air than water

  22. Clicker question • Which medium would have faster sound waves? • A) Gas (at 0 degrees) • B) Liquid (at 0 degrees) • C) Solid (at 0 degrees)

  23. Minimum and Maximum Frequency • Frequency pertains to pitch not loudness • Minimum- 20 Hertz • Below- infrasonic waves • Maximum- 20,000 Hertz • Above-ultrasonic waves • What are the corresponding wavelengths if the speed of sound in air is 343 m/s?

  24. The Doppler Effect • When there is movement involved there is an observed change in frequency • This can be seen with all types of waves

  25. The Doppler Effect • You hear a high pitch as an ambulance approaches you and as soon as it passes the pitch decreases suddenly • Compared to the noise being right beside you • It has a higher pitch as it approaches you • It is a lower pitch when it moves away from you

  26. The Red Shift • Red light has the longest wavelength (colors) • When we look at other galaxies we notice that their colors are shifted towards red • This is due to the Doppler effect • What does this indicate? • The universe is expanding

  27. Resonance • Objects/Substances can have a natural frequency in which it will begin to vibrate when that frequency strikes the object • If another object begins emitting that wave at that natural frequency then the second object will begin to vibrate at the same frequency • Example opera singer shattering glass, musical cups

  28. Standing Waves/Harmonics • On a string standing waves are made • b-1st harmonic • natural frequency • c-2nd harmonic • 1st overtone • One octave higher • d-3rd harmonic • 2nd Overtone

  29. Waves Electromagnetic Waves and Light

  30. Types of Waves • Mechanical Waves • Use a Medium • Traverse (ocean waves) and Longitudinal (springsand sound) • Electromagnetic Waves • Do not require a medium • Are transverse waves • Are formed when an electric field is coupled with a magnetic field • Light, microwaves, radiowavesetc

  31. Electromagnetic Spectrum

  32. Visible Light • White light is a mixture of different colors • Red light has the longest wavelength (700 nm) • Violet has the shortest (400 nm) • ROYGBIV

  33. EM Radiation • Is emitted from energized matter • After energy is absorbed by matter it is emitted as EM radiation

  34. Speed of Light • The speed of light=c • The speed of light is 3.0 x 108 m/s in a vacuum • IF there is a medium the speed will depend on the type of medium • Denser=slower

  35. Example • The sun is 1.50 x 108 km from Earth. How long does it take for the light from the Sun to reach us? • d=1.50 x 108 km • d=1.50 x 1011 m • c=3.0 x 108 m/s • t=? • v=d/t

  36. Example • What is the frequency of red light if the wavelength is 700 nm? • λ=700 nm • λ=7.0 x 10-9 m • c=3.0 x 108 m/s • f=? • v=λf • c=λf

  37. Waves, Sound and Light Snell’s Law

  38. Wave Speed • Wave Speed depends on the medium • When waves travel from one medium into another the speed will change • As the waves moves to more shallow water the waves slow down

  39. Wave Speed • Waves traveling perpendicular to the new medium (θi=0 or angle of incidence) continue in the same direction • Velocity changes, but the frequency stays constant • Soo.. Wavelength will change • When waves are not perpendicular they will bend

  40. Refraction • Refraction-bending of a wave due to changing medium • Optical Density- a measure of how difficult it is for light to travel in a substance • Vacuum<air<water<glass<diamond

  41. Angles • Angle of Incidence • Angle between the incident ray (original wave/ray) and the normal • Angle of refraction • Angle between the refracted ray and the normal

  42. Snell’s Law • When light travels from • Less dense to more dense medium it slows down and bends towards the normal • More dense to less dense medium it speeds up and bends away from the normal

  43. Snell’s Law • nisinθi=nrsinθr • ni=index of refraction (first medium) • θi=angle of incidence • nr=index of refraction (second medium) • θr=angle of refraction

  44. Index

  45. Example • A ray of light traveling in air strikes a block of quartz at an angle of 15 degrees. Find the angle of refraction. Draw a diagram.

  46. Example • A ray of light travels from underwater into air. It travels in the air at an angle of 65 degrees, find the incident angle. Draw a diagram.

  47. Index of Refraction • n=c/v • n=index of refraction • c=speed of light in vacuum 3.0 x108 m/s • v= speed of light in substance

  48. Example • What is the speed of light in water? • c=3.0 x 108 m/s • n=1.33 • v=? • n=c/v

  49. Total Internal Reflection • When passing from a more dense to a less dense medium, light reflects away from the normal • If the angle of large enough then the angle of refraction will be parallel to the medium boundary (θr=90 degrees)

  50. Critical Angle • Critical Angle- θi that results in θr = 90 • Total internal reflection- the wave does not pass into the next medium, occurs when θi>θc

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