Thin Lens Approximation
For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens. For a converging lens (for example a 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 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.
Read more about this topic: Focal Length
Famous quotes containing the word thin:
“I remember how they picked me up, a spindly kid,
Pinching and poking my thin ribs
Till I lay in their laps, laughing,
Weak as a whiffet;”
—Theodore Roethke (19081963)