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).