Focus (optics)

The focus or focal point is that a special point imaging optical assembly in which intersect rays incident parallel to the optical axis. It is located on the optical axis and in the focal plane perpendicular (or sometimes Fourier plane ). In this level rays incident parallel to each other, but not necessarily parallel to the optical axis intersect. For distant objects, as the sun, the focal plane is the image plane at the same time - the hot image of the sun is the focal point of his name.

The weaker will focus the lens, the more the focus is removed, see the focal length and the principal plane. For diverging lenses or convex mirrors ( negative power ), it is the rearward extensions of broken or reflected, divergent rays that intersect at the focus.

A lens or lens group has a second focus: rays emanating input side of this point (or seem outclassed in negative refractive power on these ), the output side are parallel to the optical axis, see collimator and eyepiece of the Dutch telescope. A parabolic mirror on the other hand has only one focal point.

The mathematical foci of an ellipse and a hyperbola are indeed no foci within the meaning of this article, but they are derived from the physical focal point, as well as real ellipsoids and paraboloids light rays in the (mathematical) can focus focus eg the parabolic reflector or parabolic antenna.