# Lenses

Thin Lenses

• "thin" compared to other sizes or dimensions or lengths in the situation

• We will ignore the thickness of the lenses

As with mirrors, the axis of symmetry of a lens is called the Optic Axis.

Consider rays of light that come into a lens parallel to this Optic Axis. These could be rays of light that originated from an infinitely far distant object.

When these parallel rays pass through a converging lens, they converge on a point and then diverge as they go on their way. This point, through which the rays all pass, is called the focal point F. Since we can turn the lens around and it behaves the same way, there is a focal point on both sides of the lens. The distance from the lens to the focal point is called the focal length and is labeled f. The focal length is positive for a converging lens.

When these parallel rays pass through a diverging lens, they diverge from a point on the other side of the lens. This point is called the focal point F. Since we can turn the lens around and it behaves the same way, there is a focal point on both sides of the lens. The distance from the lens to the focal point is called the focal length and is labeled f. The focal length is negative for a diverging lens.

As with mirrors, we will concentrate on three principal rays in our understanding of image formation.

 Apparent Depth Images Formed by Lenses Return to Ch 18, Mirrors and Lenses