Editing Focal length (section)

== Thin lens approximation ==
For a thin lens in air, the focal length is the distance from the center of the [[Lens (optics)|lens]] to the principal foci (or ''focal points'') of the lens. For a converging lens (for example a [[Lens (optics)#Types of simple lenses|convex lens]]), the focal length is positive and is the distance at which a beam of [[collimated light]] will be focused to a single spot. For a diverging lens (for example a [[Lens (optics)#Types of simple lenses|concave lens]]), the focal length is negative and is the distance to the point from which a collimated beam appears to be diverging after passing through the lens.

When a lens is used to form an image of some object, the distance from the object to the lens ''u'', the distance from the lens to the image ''v'', and the focal length ''f'' are related by
:<math>\frac{1}{f} =\frac{1}{u}+\frac{1}{v}\ .</math>

The focal length of a thin ''convex'' lens can be easily measured by using it to form an image of a distant light source on a screen. The lens is moved until a sharp image is formed on the screen. In this case {{Sfrac|''u''}} is negligible, and the focal length is then given by 
:<math>f \approx v\ .</math>

Determining the focal length of a ''concave'' lens is somewhat more difficult.  The focal length of such a lens is defined as the point at which the spreading beams of light meet when they are extended backwards. No image is formed during such a test, and the focal length must be determined by passing light (for example, the light of a laser beam) through the lens, examining how much that light becomes dispersed or bent, and following the beam of light backwards to the lens's focal point.