Lenses

A lens is a transparent optical element with at least one curved surface that refracts light. Two important types are: convex lens; concave lens.

A convex lens is thicker at the centre than at the edges. It causes parallel rays to converge toward a point called the **principal focus. It is therefore called a converging lens**.

A concave lens is thinner at the centre. It causes parallel rays to spread out as though they came from a focus. It is called a **diverging lens**.

The **principal axis** is the straight line passing through the optical centre and principal focus.

The **focal length `f`** is the distance from the optical centre of a lens to its principal focus.

For appropriate sign conventions: **1f = 1v + 1u** where: `f` = focal length; `u` = object distance; `v` = image distance.

Magnification can be written as: m = image height / object height and, with an appropriate convention: **m = vu**

Depending on object position, a convex lens can produce: * real or virtual images; * enlarged, reduced, or same-size images; * inverted or upright images.

For a real object, a concave lens normally produces a: * virtual; * upright; * reduced image.