the lens equations

• d - distance from either object (o) or image (i) from optical centre

  h - height of either object or image

  f - distance from the principle focus to the optical centre
• The thin lens equation

  $1/d(o) +$$1/d(i) =$$1/f$

  keep in mind:

  • object distances are always positive
  • image distances are positive for real images and negative for virtual images
  • the focal length is positive for converging lenses and negative for diverging lenses
• the magnification equation

  $M =$$h(i)/h(o) =$$-d(i)/d(o)$

  keep in mind:

  • object and image heights are positive when measured up from the principle axis and negative when measured down from the principle axis
  • magnification is positive for an upright image and negative for an inverted image