Lenses

Lenses

Convex lenses bring rays together.

Concave lenses spread rays apart.

The position of the object determines what kind of image is formed.

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.

Convex 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.

Concave 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.

Principal Axis

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

Focal Length

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

Lens Formula

For appropriate sign conventions:

**1f = 1v + 1u**

where:

0 = focal length;

0 = object distance;

0 = image distance.

Magnification

Magnification can be written as:

m = image height / object height

and, with an appropriate convention:

**m = vu**

Convex-Lens Images

Depending on object position, a convex lens can produce:

real or virtual images; enlarged, reduced, or same-size images; * inverted or upright images.

Concave-Lens Image

For a real object, a concave lens normally produces a:

virtual; upright; * reduced image.

Lenses — Advanced

Lens behaviour follows refraction at two curved interfaces.

The thin-lens approximation allows image formation to be described using focal length, object distance, image distance, and magnification.