OS3 Wave Properties/Models

1. OS3 Wave Properties/Models 1

2. Model-Something used to represent something else • Can be bigger, smaller or same size • Models have limitation-never have the exact same properties as the real object

3. Models of Light • Wave model • Particle model

4. Wave model-thought of as a traditional wave that can transfer energy • Pros: Waves reflect, refract, have wavelengths and frequencies • Cons: Wave needs a medium and can NOT travel through a vacuum-light can

5. Particle Model-Light thought of as particles • Pros: particles reflect, affected by gravity, change speed in a new medium, have color, do not need a medium • Cons: particles do not refract (bend) according to Snell’s Law

6. Scientists invented their own model for light called a photon • Photon-Particle-like bundle of energy that moves like a wave • Contains both particle and wave properties

7. Wave- A disturbance in a medium • A true wave must have a medium to pass through • Examples of mediums: Air, water, metal, gas, etc.

8. Waves transfer energy from one place to another with little transfer of the medium itself

9. 1. Transverse Waves • 2. Longitudinal Waves

10. Transverse Waves-The direction of propagation (motion) of the wave is perpendicular to the disturbance • Examples: Water waves, light waves, radio waves, stretched strings of musical instruments

11. motion Disturbance • Longitudinal Waves-The propagation is parallel to the disturbance • Example: Sound waves Compression- air molecules squished together Rarefaction- molecules spread out

12. Rest position Trough Crest Wavelength (λ) Amplitude • Crest-High point on a wave (greatest disturbance) • Trough-Low points on a wave • Rest position- The location of the medium when there is no disturbance

13. Amplitude-The distance from the rest position of a wave to the crest or trough • The maximum displacement from rest position • NOT the distance from the top of a crest to the bottom of a trough

14. Wavelength-The distance between identical points on adjacent waves • From crest to crest or trough to trough • The length of one complete wave • Represented by the symbol λ (lambda)

15. Spot Check • What letter represents the wavelength? • What letter represents the amplitude? Wavelength = A Amplitude = D

16. Spot Check • What interval represents 1 full wavelength? • Wavelenght = B-F OR A-E OR C-G

17. Frequency- The number of waves passing a point each second or how often waves are passing • NOT how fast the waves are moving • Symbol: f • Measured in waves/sec, cycles/sec, 1/sec or hertz (Hz)

18. Period- The amount of time it takes for one wave to pass a point • Period is symbolized by T • Measured in seconds

19. Frequency and period are inversely related (reciprocals) • f=1/T or T=1/f

20. Example: • Example: A pendulum makes 2 back and forth swings in 1 sec. • Frequency = 2 Hz • Period=1/2 second (time needed to complete 1 vibration)

21. Example: • Tim Ahlstrom of Oconomowoc, WI holds the record for hand clapping: 793 times in 60 seconds • Calculate the frequency • Calculate the period • Frequency = 793/60 = 13hz • Period = 1/13 = .077s

22. Bumblebees flap their wings at ~130 flaps/sec. • Produce a sound of 130 Hz • Honeybees flap their wings at 225 flaps/sec. • Produces a higher pitched sound of 225 Hz • Mosquito flaps its wings at 600 flaps/sec. • Produces a high-pitched sounds of 600 Hz

23. The speed of a wave depends on the medium (material) the wave moves through • For example: In air, all sound waves whether they have high frequency or low frequency all travel at the same speed

24. Wavelength and frequency vary inversely to produce the same wave speed for all sounds • Long wavelengths have low frequencies • Short wavelengths have high frequencies

25. In general, the more rigid the material (molecules closer together), the faster the wave moves • Example: Sound waves travel fastest in solids and slowest in gases

26. Light or Sound? • Which travels faster light or sound? • Light:3 X 108 m/s • Sound:343 m/s • Phet Simulations

27. Reflection and Transmission • http://www.kettering.edu/physics/drussell/Demos/reflect/reflect.html

28. To Recap…. • Does changing frequency affect the speed of a wave? • NO! • Does changing wavelength affect the speed of a wave? • NO! • Does amplitude affect the speed of a wave? • NO!

29. The formula for the speed of a wave is: • Wave speed = wavelength X frequency • V=wave speed (m/s or cm/s) • λ=wavelength (m or cm) • f=frequency (waves/s, cycles/s, 1/s, or Hz) V=λf

30. If a sound wave has a frequency of 396 Hz and a wavelength of 0.86 meters, what is the wave speed? • 1. List the variables • f=396 Hz • λ=0.86 m • V=? • 2. Set up the equation • V=λf • V=(0.86 m)(396 Hz) • V=340 m/s

31. Example: • Calculate the frequency of the waves. • V=λf • 2.5 = 5.0 f • f = .5 hz

32. Example: • If the frequency of a wave triples, what happens to the wavelength? λ = 1/3 • If the frequency of the wave triples, what happens to the velocity? v =same (only dependent on meduim) V=λf

33. The energy of a wave depends primarily on its frequency • The only energy problems we do will be related to the energy of light • Formula: E=hf • E=energy (Joules, J) • f=frequency (waves/s or Hz) • h=Planck’s constant (6.6 x 10-34 Js)

34. Standing wave-A wave in which parts of the wave remain stationary and the wave appears to be not traveling • Results from the interference between an incident (original) wave and a reflected one

35. Node-Any part of a standing wave that remains stationary • Antinode-The positions on a standing wave where the largest amplitudes occur • Example: Different standing waves can be produced by shaking the rope at different frequencies • Phet Simulations

36. Superposition-The adding of waves

37. Interference-When 2 or more waves overlap

38. Constructive interference (reinforcement)-Wave crests overlap to produce an increase in wave amplitude Destructive interference (cancellation)-When a crest and trough overlap, resulting in a wave of decreased amplitude

39. Interference produces beats • Beat-Result of alternate cancellation and reinforcement of 2 sound waves with slightly different frequencies

40. The vibration of an object that is made to vibrate by another vibrating object that is nearby • One object vibrates to make another object vibrate • Example: The sounding board in a musical instrument

41. The frequency that requires the least amount of energy to continue the vibration

42. Resonance-Resound or sound again • When the frequency of forced vibrations on an object match the object’s natural frequency • Results in a dramatic increase in amplitude

43. Example: Swinging When pumping you pump with the natural frequency of the swing Even small pumps or pushes from someone else will produce large amplitudes if delivered in rhythm with the natural frequency of the swinging motion

44. Compression-A pulse of compressed air • Air molecules push into their neighbors • Rarefaction-A disturbance in air in which the pressure is lowered

45. Loudness is a physiological sensation sensed in the brain • Subjective but related to sound intensity • Roughly, loudness follows the intensity decibel scale

46. Longitudinal Waves transfer energy as the disturbance is in the same direction as the wave (aLONG the wave)