The focus (from the Latin focus for "fireplace", "stove") or focal point is that special point of an imaging optical device , in which the rays intersect that are parallel to the optical axis . It is located at the distance of the focal length on the optical axis. The vertical axis at the focal point for plane is the focal plane (sometimes called Fourier plane , focal surface or focal plane ). For objects that are far away, like the sun , the focal plane is also the image plane - the focal point takes its name from the hot image of the sun, in which its radiation energy is concentrated. Here and in practice it is therefore also called a focal point . The less focused the optics , the further away the focus is.
In the case of diverging lenses or curved mirrors , it is not the refracted or reflected rays themselves, but their rear extensions that intersect in the focus. This type of focus is also called a focal point, although no radiant energy is concentrated here.
A lens or lens group has a second focal point for the rays incident from the opposite direction. A parabolic mirror, on the other hand, has the same focal point for both directions.
Although the mathematical focal points of an ellipse , parabola or hyperbola are not foci in the sense of this article, their name is derived from the physical focal point, since real ellipsoids and paraboloids can also focus light rays in the (mathematical) focal point, e.g. B. the parabolic mirror or the parabolic antenna .
- Eugene Hecht: Optik , Oldenbourg, 4th edition 2005, ISBN 3-486-27359-0