<P> Sometimes the rotating reflector is located outdoors and the reflected sunlight passes through an opening in a wall into an indoor kitchen, often a large communal one, where the cooking is done . </P> <P> Paraboloidal reflectors that have their centres of mass coincident with their focal points are useful . They can be easily turned to follow the sun's motions in the sky, rotating about any axis that passes through the focus . Two perpendicular axes can be used, intersecting at the focus, to allow the paraboloid to follow both the sun's daily motion and its seasonal one . The cooking pot stays stationary at the focus . If the paraboloidal reflector is axially symmetrical and is made of material of uniform thickness, its centre of mass coincides with its focus if the depth of the reflector, measured along its axis of symmetry from the vertex to the plane of the rim, is 1.8478 times its focal length . The radius of the rim of the reflector is 2.7187 times the focal length . The angular radius of the rim, as seen from the focal point, is 72.68 degrees . </P> <P> Parabolic troughs are used to concentrate sunlight for solar - energy purposes . Some solar cookers have been built that use them in the same way . Generally, the trough is aligned with its focal line horizontal and east - west . The food to be cooked is arranged along this line . The trough is pointed so its axis of symmetry aims at the sun at noon . This requires the trough to be tilted up and down as the seasons progress . At the equinoxes, no movement of the trough is needed during the day to track the sun . At other times of year, there is a period of several hours around noon each day when no tracking is needed . Usually, the cooker is used only during this period, so no automatic sun tracking is incorporated into it . This simplicity makes the design attractive, compared with using a paraboloid . Also, being a single curve, the trough reflector is simpler to construct . However, it suffers from lower efficiency . </P> <P> It is possible to use two parabolic troughs, curved in perpendicular directions, to bring sunlight to a point focus as does a paraboloidal reflector . The incoming light strikes one of the troughs, which sends it toward a line focus . The second trough intercepts the converging light and focuses it to a point . </P>

The solar cooker works on the same principle that of