Difference between Sign conventions of mirrors and lens
Sign conventions for mirrors
- Pole (P) of mirror is taken as origin X-axis.
- Principal axis is considered as X- axis.
- Objects are always kept on left side.
- Distances parallel to the principal axis are measured from the pole of the mirror.
- Distances measured on left side are taken as negative (- X axis), whereas the distances measured towards right side are taken as positive (+ X axis).
- Height of object above and perpendicular to principal axis is taken as negative (- Y axis), and height of object below and perpendicular to principal axis is taken as positive (+ Y axis).
- The focus of concave mirror appears on left side; hence the focal length of concave mirror is taken as negative.
- The focus of convex mirror appears on right side; hence the focal length of concave mirror is taken as positive.
Sign conventions for lenses.
- Optical centre (O) of lens is taken as origin X-axis.
- Principal axis is considered as X- axis.
- Objects are always kept on left side.
- Distances parallel to the principal axis are measured from the optical centre of lens.
- Distances measured on left side are taken as negative (- X axis), whereas the distances measured towards right side are taken as positive (+ X axis).
- Height of object above and perpendicular to principal axis is taken as negative (- Y axis), and height of object below and perpendicular to principal axis is taken as positive (+ Y axis).
- The focus of concave lens appears on left side; hence the focal length of concave lens is taken as negative.
- The focus of convex lens appears on right side; hence the focal length of concave lens is taken as positive.