Physics 5.4

1. Physics 5.4

2. Waves • A vibratory disturbance that moves through a medium. • Waves can be one pulse or a periodic wave (which is just a pulse repeated many times!)

3. Which diagram does not represent a periodic wave? • 1. 2. • 3. 4. • Answer: 4 4

4. More wave stuff… • Only energy is transferred by a wave, no mass! • Mechanical waves require a medium to pass through, where as an electromagnetic wave can pass through a vacuum.

5. Longitudinal waves: • Ex/ sound waves. • The particles vibrate parallel to the motion of the wave.

6. Transverse waves: • Ex/ light waves. • The particles vibrate perpendicular to the motion of the wave.

7. Amplitude • Amplitude (A): the maximum height of the wave measured from the average height of the wave (the wave’s center). Unit: meters (m)

8. Wavelength • Wavelength (λ): The length of one wave, or the distance from a point on one wave to the same point on the next wave. Units: meters (m). In light, λ tells us the color.

9. Frequency • Frequency of oscillation (f) (or just frequency): the number of times the wave pattern repeats itself in one second. Units: seconds-1 = (1/s) = hertz (Hz) In sound, f tells us the pitch. The inverse of frequency is the period of oscillation.

10. Period • Period of oscillation (T) (or just period): duration of time between one wave and the next one passing the same spot. Units: seconds (s). The inverse of the period is frequency. Use a capital, italic T and not a lowercase one, which is used for time.

11. Lets take a look:

12. Wave Speed • The speed of the wave is denoted by velocity (v) equals the wavelength (λ) times the frequency (f). • v = λf

13. Compared to the speed of microwaves in a vacuum, the speed of x-rays in a vacuum is: • less • greater • the same • Answer:3

14. ??? • A student plucks a guitar string and the vibrations produce a sound wave with a frequency of 650 hertz. Calculate the wavelength of the sound wave in air at STP. Round your answer to the nearest hundredth of a meter. • λ = 0.51 m

15. Wave Fronts: • Flat wave fronts become spherical after passing through a lens.

16. The Doppler effect • The Doppler effect is the perceived change in frequency of sound emitted by a source moving relative to the observer: as a plane flies overhead, the note of the engine becomes noticeably lower, as does the siren noise from a fast-moving emergency vehicle as it passes. 

17. more Doppler effect…

18. An observer detects an apparent change in the frequency of sound waves produced by an airplane passing overhead. This phenomenon illustrates: • the Doppler effect • the refraction of sound waves • wave amplitude increase • wave intensity increase • Answer1

19. Interference from superposition: • Constructive interference is when two waves are in phase and the resultant wave is the sum of the two waves. Two waves 0˚ in phase create max constructive interference. • Destructive interference is when two waves are out of phase and resultant wave is smaller than both. Two waves 180˚ out of phase create max destructive interference.

20. The diagram shows two pulses of equal amplitude, A, approaching point P along a uniform string. • When the two pulses meet at P, the vertical displacement of the string at P will be • A • 2A • 0 • A/2 • Answer:3

21. ??? • Radio waves diffract around buildings more than light waves do because, compared to light waves, radio waves • move faster • move slower • have a higher frequency • have a longer wavelength • Answer4

22. Standing Wave • Standing waves are produced whenever two waves of identical frequency interfere with one another while traveling opposite directions along the same medium.

23. Node and Antinodes • Standing wave patterns are characterized by certain fixed points along the medium which undergo no displacement. These points of no displacement are called nodes (nodes can be remembered as points of no displacement). • Midway between every consecutive nodal point are points which undergo maximum displacement. These points are called antinodes.

24. A Standing Wave:

25. One end of a rope is attached to a variable speed drill and the other end is attached to a 5.0-kilogram mass. The rope is draped over a hook on a wall opposite the drill. When the drill rotates at a frequency of 20.0 Hz, standing waves of the same frequency are set up in the rope. The diagram below shows such a wave pattern. • Calculate the speed of the wave in the rope. • Answer: 60. m/s

26. Resonance • In sound applications, a resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object. • A tuning fork is struck and vibrates at a frequency of 512 Hz, and causes a second tuning fork (untouched) to vibrate at a frequency of 512 Hz.

27. Diffraction • Diffraction manifests itself in the apparent bending of waves around small obstacles and the spreading out of waves past small openings. • If the opening is larger than the wavelength of light, than there will be no diffraction. Why?

28. A beam of monochromatic light approaches a barrier having four openings, A, B, C, and D, of different sizes as shown below. • Which opening will cause the greatest diffraction? • A • B • C • D • Answer:1

29. Refraction • The bending of light when it goes from one medium (example: air) to a different medium (example: water) is called refraction.

30. The diagram shows a ray of light, R , entering glass from air. • Which path is the ray most likely to follow in the glass? • A • B • C • D • Answer3