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Definition of Terms Used For Lenses

The following technical terms occur in discussions of lenses.

  • The curvature of each surface of a lens makes it part of a sphere. The centre of curvature of the lens is the centre of this sphere.

  • The principle axis is the line through the centre of the lens, perpendicular to the lens. The lens has two surfaces – focal points and centres of curvature for both sides of the lens lie on this line.

  • The focal point or principal focus F is the point on the principal axis through which rays of light parallel to the axis are brought to a focus after passing through the lens (converging lens) or appear to be brought to a focus (diverging lens).

    As with centres of curvature, there is a focal point on either side of the lens.

    • The focal length is the distance between the centre of the lens and a focal point.

    • The distance from the centre of the lens to a centre of curvature is twice the focal length.

    • The linear magnification is the ratio between the size of the image and the size of the object, also equal to the ratio of the image and object distances:

      This is easily seen from the diagram below. The triangles OAB and BCI are similar, soand which rearranges to (1) .

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