State the working principle of a Parabolic trough collector with a neat sketch.

 A parabolic trough is a type of solar thermal collector that is straight in one dimension and curved as a parabola in the other two lined with a polished metal mirror. The energy of sunlight which enters the mirror parallel to its plane of symmetry is focused along the focal line, where objects are positioned that are intended to be heated. For example food may be placed at the focal line of a through which causes the food to be cooked when the trough is aimed so the sun is in its plane of symmetry. Further information on the use of parabolic troughs for cooking can be found in the article about solar cookers. For other purpose there is often a tube frequently a Dewar tube which runs the length of the trough at its focal line. The mirror is oriented so that sunlight which it reflects is concentrated on the tube which contains a fluid which is heated to a high temperature by the energy of the sunlight. The hot fluid can be used for many purpose. Often it is piped to a heat engine which uses the heat energy to drive machinery or to generate electricity. This solar energy collector is the most common and best known type of parabolic trough. These are used for medium and high temperature applications. Various types of concentrating solar collectors are shown in fig. It consists of a concentrating device which may be a reflecting mirror or Fresnel lenses. The beam radiation are concentrated and focussed on to the absorber.