CH1 Signalling

• Light


  Can be modelled as either rays or wave fronts
• Plane wave fronts


  Parallel to each other and do not appear to curve
• As the radius approaches infinity, the curvature approaches zero
• Converging lens


  Focuses light onto a point behind it, called the principle focus (or just focus) of the lens, f
• Power of a lens

  Measured in Dioptres, D
• Lensmaker's Equation


  1/v = 1/u + 1/f
• Linear magnification


  m = image height/object height = image distance, v/object distance, u
• CCD (Charge Coupled Device)


  Consists of a screen covered in pixels, which store charge when light falls on them
• Bit


  A unit of digital information that can have two values: high and low (or on and off)
• Resolution of an image

  The smallest distance between which two points can be distinguished, determined by the pixels used
• Image processing techniques


  • Removal of noise
  • Edge detection
  • Increasing contrast
  • Changing brightness
• Analogue signal


  Continuously varies between values
• Digital signal


  Only takes discrete values
• Sampling


  Converting an analogue signal into a digital signal by taking regular measurements and rounding to the nearest quantisation level
• Quantisation error


  Difference between the actual value and the quantisation level it has been rounded to
• Resolution of a sample

  p.d. range of signal/number of quantisation levels
• Aliasing


  Production of a spurious (false) low frequency signal from a high frequency signal when it is sampled too infrequently
• Electromagnetic spectrum


  Group of transverse waves with oscillating electric and magnetic fields
• Polarisation


  EM waves vibrating in one plane only
• Kirchoff's first law


  Conservation of charge - the total current at a junction must remain the same
• Resistance


  Ratio of potential difference to current, R = V/I
• Conductance


  Reciprocal of resistance, G = 1/R = I/V
• Conductors, semiconductors, insulators


  • Conductors have high proportion of mobile charge carriers
  • Semiconductors have low proportion of mobile charge carriers
  • Insulators have no (or very few) mobile charge carriers
• Potential divider


  Potential difference in a series circuit is split between components in the ratio of their resistances
• EMF (Electromotive Force)


  Energy provided by the source per unit charge
• Kirchoff's second law


  The sum of all the voltages across the circuit must be equal to the EMF of the circuit