Question

Write the Cartesian sign convention for lenses.

Answer 1

In this instance, the major axis of the lens is assumed to be the X-axis of the coordinate system, and the optical center (O) of the lens is assumed to be the origin.

1. The item is positioned to the left of the lens at all times.
2. The optical center of the lens is used to measure all distances parallel to the primary axis.
3. Distances measured to the left of the origin are interpreted as negative, and all distances measured to the right of the origin as positive.
4. Positive distances are those that are measured above and perpendicular to the primary axis.
5. Negative distances are those that are measured below and perpendicular to the primary axis.
6. Convex lenses have positive focal lengths, while concave lenses have negative focus lengths.