 The selective surface consists of a sheet of metal foil glued to the outside surface of the wall and it absorbs almost all the radiation in the visible portion of the solar spectrum and emits very little in the infrared range.High absorbency turns the sunlight into heat at the wall's surface, and low emittance prevents the heat from radiating back towards the glass.• Although the Trombe walls are usually made of solid materials, such as concrete, brick, stone, or adobe, they can also be made of water.