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  1. 1. WAVES Physics Notes GCE Study Buddy
  2. 2. Waves • Waves are repeated to-and-fro vibrations that transfer energy away from an energy source
  3. 3. Describing wave motion Term Description SI units Amplitude, A The maximum displacement of the rope from the rest position Metre (m) Wavelength, λ The shortest distance between 2 successive crests or troughs Metre (m) Frequency, f The number of complete waves produced per second Hertz (Hz) Period, T The time taken to produce one complete wave Second (s) Wave speed, v The distance travelled by a wave in 1 second Second (s)
  4. 4. Describing wave motion crest trough
  5. 5. Types of Waves • Transverse waves o The vibration of the particles in the medium is perpendicular to the direction in which the wave travels o Eg. water waves, rope waves, all types of electromagnetic waves including light waves, microwaves, X-rays, gamma rays o The highest point reached by a vibrating particle in a transverse wave is called crest or peak while the lowest point is called trough • Longitudinal waves o The vibration of the particles in the medium is parallel to the direction in which the wave travels o Eg. sound waves o The section in which the vibrating particles in a longitudinal wave are closest together is called compression while the section in which the vibrating particles are furthest apart is called rarefaction
  6. 6. Longitudinal and Transverse waves
  7. 7. Wavefronts • Any line or surface over which all the vibrating particles are in the same phase • Particles in the same phase have the same speed and are at equal distances from their source • In transverse waves, wavefronts are normally lines joining all the peaks at equal distance from their source • The distance between successive wavefronts equals a wavelength • The direction of travel of a wave is always perpendicular to its wavefronts as indicated by lines drawn perpendicular to the wavefronts.
  8. 8. Wavefronts
  9. 9. Wave Equation • Velocity of wave, v = fλ Example: The speed of light in vacuum is 3 x 108 m/s Calculate the frequency of orange light, given that its wavelength in vacuum is 6 x 107 m. 3 x 108 = f x 6 x 10-7 f = (3 x 108)/(6 x 10-7) = 5 x 1014 Hz
  10. 10. Ripple Tank • The properties of waves in general and water waves in particular are most easily studied in a ripple tank
  11. 11. Reflection of waves • Waves are reflected when an obstacle is placed in their paths • All reflected waves obey the law of reflection which states o The angle of reflection equals the angle of incidence o The incident wave, the reflected wave, and the normal all lie on the same plane
  12. 12. Properties of reflected waves • The reflected wave the same wavelength, frequency, and speed as the incident wave • The velocities of the reflected and incident waves are different because they travel in different directions • The angle of reflection equal to the angle of incidence
  13. 13. Refraction of waves • Waves are refracted when their speeds are changed • The speed of a wave is changed when the wave moves from a dense medium into a less dense medium or from deep water to shallower water • If the incident wave is travelling along the normal, it will continue to travel along the normal after entering water of a different depth • In all other cases, refraction produces a change in wave direction • On entering shallower water, the wave direction bends towards the normal. • On entering deeper water, the wave direction bends away from the normal
  14. 14. Refraction of water waves
  15. 15. Refraction of waves Properties Shallower to deeper water Deeper to shallower water Wavelength Increases Decreases Frequency Unchanged Unchanged Speed Increases Decreases Velocity Increases Decreases Direction of travel Bends away from normal Bends towards normal
  16. 16. Ripple Tank to show refraction of waves
  17. 17. Sound • Production of sound waves by vibrating sources: sound is produced by vibrating sources (eg tuning fork) placed in a medium (solid, liquid, gas) • Nature of sound waves o It is a form of energy that can be transferred from one point to another o It is an example of longitudinal waves consisting of compressions and rarefactions o Compressions are regions where air pressure is slightly higher than he surrounding air pressure o Rarefactions are regions where air pressure is slightly lower than the surrounding air pressure
  18. 18. Sound waves
  19. 19. Range of audible frequency • The range of frequency that a human ear can detect is from 20 Hz to 20,000 Hz
  20. 20. Transmission of sound in a medium • Sound waves require a medium for transmission • Sound waves cannot travel through a vacuum Vacuum jar
  21. 21. Speed of sound in solid, liquid, gas Medim Speed in m/s Air (gas) 300 Water (liquid) 1500 Iron (solid) 5000 Speed of sound changes with changes in temperature or humidity Change in Explanation Temperature Sound travels faster when temperature rises Humidity Sound travels faster when humidity increases Pressure A change in pressure does not affect speed of sound
  22. 22. Experiments to determine speed of sound in air • Pistol method o Observer A and B are positioned at a distance s apart and with a measuring tape, measure and record s. (must be more than 1km) o A fires a starting pistol o B starts the stopwatch on seeing the flash of the pistol and stops the stopwatch when he hears the sound o The time t, is recorded o The speed of sound v can be calculated by • Speed = distance / time • For better accuracy, the experiment should be repeated and the average speed of sound can be calculated. • The experiment can be repeated by interchanging the positions of A and B so as to minimise the effect of the wind direction.
  23. 23. Experiments to determine speed of sound in air • Echo method o Observer A and B are positioned at a distance s from the wall and with a measuring tape, measure and record s o A claps two wooden blocks. o On hearing the echo (reflected from the wall), he repeats the clap o B starts the stopwatch and also starts counting from zero till the nth clap. o The time interval tn is recorded o The average time between successive claps is t = t n/n o The speed of sound v can be calculated by • speed = distance/time
  24. 24. Reflection of sound • An echo is a reflection of sound • Reverberation is the effect of a prolonged sound due to the merging of many echoes • Echoes are used in determining the depth of sea and locations of shoals of fish
  25. 25. Electromagnetic spectrum • The entire possible range of electromagnetic waves is called the electromagnetic spectrum
  26. 26. Properties of electromagnetic waves • They are generated by accelerating charged particles • They travel at 3 x 108 m/s in vacuum • They obey the laws of reflection and refraction • They show wave properties such as diffraction and interference • They obey the equation v = fλ • They are progressive transverse waves carrying energy in the form of oscillating electrical and magnetic fields vibrating at right angles to one another and to the direction of travel of the waves
  27. 27. Radiation Wavelength Sources /m Detectors Uses Gamma rays 10-15 – 10-11 Cosmic rays radioactive substances, nuclear changes G M tubes with counters, bubble/cloud chambers Checking welds, killing cancer cells in radiology, photography X-rays 10-13 – 10-8 X-ray tubes: stopping of fast-moving electrons by a heavy metal target Photographic film, fluorescent screen X-ray photography, analysis of crystal structure Ultraviolet 10-8 – 10-7 Mercury vapor lamps, sun, spark discharges Fluorescent screens/dyes Forgery detection, sun lamps Visible light 10-7 Hot bodies, lasers, fluorescent screens, sun Photographic film, photodiodes Chemical spectral analysis, fibre optics Infra-red 10-7 – 10-3 Warm bodies, sun, fires, furnaces Blackened thermometers, thermocouples TV remote control, radiant heaters VHF and UHF (TV) waves 10-4 – 10-1 TV transmitters Aerials and TV sets Communications, entertainment Radio waves 10-3 – 103 Radio transmitters Aerials and radio sets Radio, telescope, radar, communications
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