Parabolic reflector

A parabolic mirror is a paraboloid of revolution as a reflecting surface. A simple curved parabolic reflector is a parabolic trough. Parabolic mirrors reflect incident plane waves so that they converge to a point ( focal point).

Parabolic function largely independent of the type of wave. The only requirement is that the mirror is large compared to the wavelength, and the wave is reflected at the surface. They are suitable for electromagnetic waves such as light, radar or radio waves. Also, sound waves can be focused at a focal point or radiate from there in planar form with parabolic mirrors.

Parabolic mirrors have similar imaging characteristics such as lenses, but they are only suitable for light, since they are not penetrated by other waves in general.

Focal property of parabolic surfaces

A parabola can be defined as follows:

" A parabola is the locus of all points P whose distance to a special fixed point - the focal point F - equal to a specific line - the directrix l -. 's " ( See chart)

The perpendicular bisector PG is the tangent to the parabola at P. It forms with the PF beam and the normal incident beam at the same angle, so the latter is reflected in the direction of F, the focal point.

From the parabola is by rotation about its axis of symmetry, the paraboloid of revolution, the reflective surface of a parabolic mirror.

A small area of the parabola on either side of the vertex A is approximately a circle. This is the area emanating from the point F solid angle, which is used by spherical concave mirrors (or spherical mirrors ). Such mirrors are easier to produce and therefore cheaper than parabolic mirrors. They bundle incident parallel light approximately in one point. If they are used as reflectors for ( point-like ) light source for producing a parallel beam of light, their smaller solid angle is disadvantageous because less light is detected.

Applications

Parabolic mirrors are both to receive (eg: Satellite TV, mirror telescopes ) used by waves from a certain direction and to send ( eg, radio, radar ) waves in a particular direction. Often with a single parabolic mirror is both sent and received ( time division multiplexing ).

Light

In the headlights radial rays of light of a point source of light are reflected by parabolic mirrors so that a large part of the radiation leaking approximately parallel.

The mirrors of reflecting telescopes are some types of parabolic mirrors. They serve the (parallel ) light from stars and other astronomical objects observation to focus so much so that its intensity is sufficient to be seen with the eye or measured in detectors. The best known example are the mirrors of Newtonians with a focal length compared to the large mirror diameter, that is a "fast" focal ratio. From a ratio of about 8:1, a parabolic mirror and a (simple -make ) spherical mirror are so similar that for cost reasons in some Newtonians no parabolic mirrors are used. The mirror telescopes used today in large-scale research are mostly Ritchey -Chrétien Cassegrain telescopes have the hyperbolic mirror surfaces.

Another important application is the clustering of sunlight for harnessing solar energy. By bundling with large parabolic mirrors can be at the focal point reach high temperatures. The so available energy can be used to melt metals (see solar furnace ) or steam to produce (see solar thermal power plant ). Even small-scale applications such as solar cookers often make use of parabolic mirrors for concentrating solar energy. This principle has also been used in ancient times for the inflammation of the Olympic torch, a torch was lit at the focal point.

Radar and radio waves

Radars with long range are equipped with parabolic mirrors to give the radar beam has a good directivity.

In radio astronomy, extremely large parabolic mirror made ​​of metal are used which are similar to the levels in radars.

When radio links are used for both the transmitter and receiver for the parabolic mirror in the form of satellite dishes. In this way can be made with relatively little transmit power a communication link over long distances.

The receiving antennas for satellite television are also parabolic antennas.

Sound

In Flüstergewölben the walls are approximately parabolic in shape. In this way, soft sounds at the focal point are transmitted over long distances without great loss.

A microphone at the focal point of a parabolic mirror receives sound from the direction of the axis of reflection. Noise coming from other directions are received only weak. This design is suitable for example as a directional microphone.