Page 39 - cbse-class-10-light-reflection-and-refraction

 All distances on the principal axis are measured from the optical center.

 Distances measured in the direction of incident rays are positive, while
distances measured in the opposite direction of incident rays are negative.

 All measurements taken above the principal axis are positive. As a result, the
height of an object and the height of an erect image are both positive, while
all distances measured below the principal axis are negative.

Note:

The rules are the same as for spherical mirrors.

The sign convention for lenses is shown in the table below:

For measuring various distances, the following sign convention is used:

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