2. Sign Convention of Mirror, Mirror Formula and Magnification

Sign Conventions of Spherical Mirror

• All the distances are measured from the pole of the mirror as the origin.
• Distances measured in the direction of incident rays are taken as positive.
• Distances measured opposite to the direction of incident rays are taken as negative.
• Distances measured upward and perpendicular to the principal axis are taken as positive.
• Distances measured downward and perpendicular to the principal axis are taken as negative.
  1f=1v+1u …where f, v and u are focal length, image distance, object distance

Rules for reflection of rays by spherical mirrors

(i) A ray of light from the object, parallel to the principal axis, after refraction from a convex lens, passes through the principal focus on the other side of the lens. In case of a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens.

 (ii) A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis. A ray of light appearing to meet at the principal focus of a concave lens, after refraction, will emerge parallel to the principal axis.

(iii) A ray of light passing through the optical centre of a lens will emerge without any deviation. 

Mirror formula

where, v = Image distance
u = Object distance
f = Focal length

Magnification
The size of an image relative to the size of the object creating it. 
Linear magnification refers to the ratio of image length to object length measured in planes that are perpendicular to the optical axis. A negative value of linear magnification denotes an inverted image.

 Magnification of Spherical Mirrors

where, h' = Height of the image
and h = Height of the object
Note -The height of the image should be taken as positive for virtual images. However, it is to be taken as negative for real images.