waves

• Waves


  Transfer energy and information without transferring matter; the particles oscillate about a fixed point
• Transverse waves


  • Have peaks and troughs
  • Vibrations are at right angles to the direction of travel
  • An example is light

  
• Longitudinal waves


  • Consists of compressions (particles pushed together) and rarefactions (particles moved apart)
  • Vibrations are in the same direction as the direction of travel
  • An example is sound
• Amplitude


  The distance from the equilibrium position to the maximum displacement
• Wavefront


  A line joining points on a wave at the same point in their wave cycle at a given time

  
• Frequency


  The number of waves that pass a single point per second
• Wavelength


  The distance between a point on one wave and the same point on the next wave
• Time period


  The time taken for one complete wave to pass a fixed point
• Wave speed


  Speed = Frequency x Wavelength

  
• Frequency


  The reciprocal of the time period, measured in Hertz (Hz)
• Doppler effect


  If a wave source is moving relative to an observer, there will be a change in the observed frequency and wavelength

  
• Reflection


  • All waves can be reflected when they travel from a medium of low optical density (such as air) to one of much higher optical density (such as glass)
  • Angle of incidence = angle of reflection
  • Frequency, wavelength, and speed are all unchanged

  
• Refraction


  • All waves can be refracted, which is when the speed of a wave changes when it enters a new medium
  • If the wave enters a denser medium, its speed decreases and it bends towards the normal
  • If the wave enters a less dense medium, its speed increases and it bends away from the normal
  • Frequency stays the same but the wavelength changes

  
• Electromagnetic spectrum


  • Radio waves
  • Microwaves
  • Infrared radiation
  • Visible light
  • Ultraviolet light
  • X-rays
  • Gamma radiation

  
• All electromagnetic waves travel with the same high speed in a vacuum and approximately the same speed in air
• Uses of electromagnetic waves


  • Radio waves for radio and television communications
  • Microwaves for satellite transmissions and cooking
  • Infrared radiation for heaters and night vision equipment
  • Visible light for fibre optics and photography
  • Ultraviolet light for fluorescent lamps
  • X-rays for medical imaging and security
  • Gamma radiation for sterilising food and medical equipment

  
• Hazards of electromagnetic waves

  • Microwaves can cause internal heating of body tissues
  • Infrared radiation can cause skin burns
  • Ultraviolet light exposure increases the risk of skin cancer and blindness
  • X-rays and Gamma rays are ionising radiation that can cause mutations leading to cancer
• Light waves


  • Transverse waves
  • Can be reflected and refracted

  
• Reflection of light


  Light reflects at a plane mirror and forms an image

  
• Refraction of light


  1. When light enters a more optically dense medium, the angle of incidence is greater than the angle of refraction

  2. The opposite is true when light enters a less optically dense medium

  
• Snell's law


  Relates the angle of incidence and the angle of refraction to the refractive index of a medium

  
• Total internal reflection


  • At a certain angle of incidence called the critical angle, the light will travel along the boundary between the two media
  • Total internal reflection occurs when the angle of incidence is greater than the critical angle and the light reflects back into the medium
  • For total internal reflection to occur, the light must be travelling from a more optically dense medium into a less optically dense medium

  
• Critical angle


  The angle of incidence at which total internal reflection occurs

  
• Optical fibres


  • Long thin rods of glass surrounded by cladding which use total internal reflection to transfer information by light, even when bent
  • Used extensively in medicine and communications

  
• Sound waves


  • Longitudinal waves
  • Can be reflected and refracted
• Audible frequency range


  20 Hz to 20000 Hz for a healthy human ear
• Measuring speed of sound in air


  Make a noise at a known, large distance from a solid wall and record the time for the echo (reflected sound) to be heard, then use speed = distance/time
• Oscilloscope


  Can be used to display a sound wave and find its frequency and amplitude
• Amplitude of sound wave

  The greater the amplitude, the louder the sound
• Frequency of sound wave


  The greater the frequency, the higher the pitch
• Sound wave characteristics


  • Quiet and low pitched
  • Loud and low pitched
  • Loud and high pitched