A greenhouse uses a special kind of glass that acts as a medium which selectively transmits spectral frequencies. Spectral comes from the word "spectrum".In layman's terms, a spectral frequency can be defined in terms of the following principle: any object in the universe emits, radiates or transmits light. The distribution of this light along an electromagnetic spectrum is determined by the object's composition.Therefore, the glass of a greenhouse traps energy within the greenhouse and the heat in turn provides heat for the plants and the ground inside the greenhouse. It warms the air near the ground, preventing it from rising and leaving the confines of the structure.For example, if you open a small window near the roof of a greenhouse, the temperature drops significantly. This is because of the auto vent automatic cooling system. An autovent is simply a device used by greenhouses that maintain a range of temperatures inside. This is how greenhouses trap electromagnetic radiation and prevents convection (transference of heat by currents within a fluid).Curious about how the idea of a greenhouse came about? It goes back to the days of the Romans, who - as history annals show - were the first people to create a structure to protect plants. Using heated pits, they put up slabs of rock to form primitive greenhouses. The term "glasshouse" which is the correct name of this structure, was adopted sometime in the 17th and 18th centuries.At that time, however, the error was in believing that heat was more important than light for plants to thrive. Structures were being built to exclude the entry of light, but by the time the glass tax of 1845 was abolished, the design of greenhouses started to change.Builders realized then that a curved roof instead of a flat one allowed higher concentrations of the sun's rays, and that by using iron instead of wood, the greenhouse could be structurally reinforced and made capable of absorbing more light.