Diopter

• The power of a lens is conventionally expressed in the unit of diopter (D).
• The unit of diopter is simply inverse meters.
• The power of the lens is the inverse of the focal length: D = 1/f
• The mirror equation for lenses can thus be expressed as \[ \rm \mathbf{ \frac{1}{p} + \frac{1}{q} = D,} \]

  but you have to make sure you are using meters in your calculations.

• Optometrists use
  • positive (D > 0) for converging lenses (lenses with a positive focal length), and
  • negative (D < 0) for diverging lenses (lenses with a negative focal length).

  © MultiMedia Physics 2000