FOCAL POINT - FOCAL LENGTH - REFRACTING POWER

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      Focal point: (Focus point)
               When light rays incident on a lens parallel to the principle axis of a lens, focus at a point. the point is called focus point or focal point.

      Focal length:  Distance between focal point and optical center of a lens is called focal length. 

      Optical power:(Refracting power)
      Reciprocal of the focal length is called optical power. SI units are diopter or 1/meter.

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