# Principal Rays for Lenses

Just as in the case of a mirror, there are three principal rays that can be used to establish the position of images for lenses:

1. Ray parallel to the axis on left will pass through the right focal point.
2. Ray parallel to the axis on right will pass through the right focal point.
3. Ray passing through the center will not be deflected.

Conventions:

• The focal length of a converging (convex) lens is positive
• The focal length of a diverging (concave) lens is negative.
• The image distance, q, is positive, if the image is formed on the opposite side of the lens, and negative, if it is formed on the same side as the object.

With these conventions the mirror equation also applies to lenses:

$\rm \mathbf{ \frac{1}{f} = \frac{1}{p} + \frac{1}{q}}$