Evacuated tubes operate on the basic principle of using light from the sun to heat liquids. Each evacuated tube consists of two glass tubes made from extremely strong special glass. The outer tube is transparent allowing light to pass through with minimal reflection.
The inner tube is coated with a special selective coating and converts solar radiation into thermal energy. The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperatures.
This “evacuation” of the gasses forms a vacuum.