WA2 Revision. Light (TIR and Lens), Waves part 1 & 2

• Frequency, f

  • Number of complete oscillations per second
  • Formula : f = 1/T
• Wavelength, λ

  Shortest distance between two consecutive crests or troughs or any two points which are in phase
• Period, T

  Time taken for the wave to produce one complete wave
• Amplitude, A

  Maximum displacement of the wave from the rest position
• Wavefront

  Imaginary line that joins all the points that are in phase
• Types of waves

  • Transverse waves
  • Longitudinal waves
• Transverse waves

  • Waves which travel in a direction perpendicular to the direction of the vibrations
• Longitudinal waves

  • Waves which travel in a direction parallel to the direction of vibrations
• When travelling through different mediums

  • Speed of light waves decreases in optically denser medium
  • Speed of sound increases in a denser medium
• Refractive Index, η

  Formula : Speed of light in vacuum / speed of light in medium
• Refractive index

  = sin(angle of incidence) / sin(angle of refraction)
• Total internal reflection

  Occurs when light rays travel from an optically denser medium to an optically less dense medium and the angle of incidence in the denser medium is greater than the critical angle
• Critical angle

  Angle of incidence in the optically denser medium that results in an angle of refraction of 90° in the optically less dense medium
• Total internal reflection is used in optical fibres in telecommunication and medical use such as laparoscopy
• Advantages of optical fibres over copper wires include higher carrying capacity, less signal degradation, light weight and lower costs
• Speed of light in vacuum = 3 x 10^8 m/s
• Focal point

  Point where a parallel beam of light will converge at
• Focal length

  Distance between the centre of the lens and the focal point
• Production of sound by vibrating sources

  • A vibrating object in a medium causes shifting of layers of the particles in the medium resulting in a series of compressions and rarefactions
  • Longitudinal sound waves are produced and propagated through the air
  • Direction of vibration of the medium's particles is parallel to the direction the wave travels
• Compressions

  Regions where air pressure is slightly higher than the surrounding air pressure
• Rarefactions

  Regions where air pressure is slightly lower than the surrounding air pressure
• Speed of sound

  Fastest in solid, then liquid, slowest in gas
• Sound waves also undergo reflection to produce echoes
• Loudness

  Related to amplitude of sound wave
• Pitch

  Related to frequency of sound wave
• Range of audibility for humans is from 20 Hz to 20,000 Hz
• Ultrasound

  Sound with frequencies above the upper limit of the human range of audibility
• Uses of ultrasound

  • Quality control
  • Prenatal scanning
• Ultrasound is used to examine foetal development instead of X-rays because it does not harm the foetus
• Methods to determine speed of sound

  • Method 1: Computer detects pulses from two microphones at different timings
  • Method 2: Measure time between seeing spark and hearing sound
  • Method 3: Measure time between hitting planks and hearing echo
• Wave speed = frequency x wavelength
• Formula for echoes : v = 2d/t