CBSE Class 10 Answered

Sign convention for reflection by Lens

1. The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.
2. All distances parallel to the principal axis are measured from the pole of the mirror.
3. All distances measured to the right of the origin (along +x-axis) are taken as positive, while those measured to the left of the origin (along −x-axis) are taken as negative.
4. Distances measured perpendicular to and above the principal axis (along +y-axis) are taken as positive.
5. Distances measured perpendicular to and below the principal axis (along −y-axis) are taken as negative.
6. According to the sign convention, the focal length of a convex lens is positive and that of a concave lens is negative.

Note: The sign conventions for all types of lenses is the same. Infact the sign convention for spherical mirrors and lenses is also same (Except for the focal length).