# LIGHT – REFLECTION AND REFRACTION

## Spherical Mirrors

## Sign Convention for Reflection by Spherical Mirrors

While dealing with the reflection of light by spherical mirrors, we shall follow a set of sign conventions called the New Cartesian Sign Convention. In this convention, the pole (P) of the mirror is taken as the origin. The principal axis of the mirror is taken as the x-axis (X’X) of the coordinate system. The conventions are as follows –

- 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.
- All distances parallel to the principal axis are measured from the
pole of the mirror.
- All the 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.
- Distances measured perpendicular to and above the principal axis
(along + y-axis) are taken as positive.
- Distances measured perpendicular to and below the principal axis
(along –y-axis) are taken as negative.

The New Cartesian Sign Convention described above is illustrated in Fig.10.9 for your reference.

Figure 10.9 The New Cartesian Sign Convention for spherical mirrors

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