A greenhouse in Saint Paul, Minnesota
A greenhouse is a building where plants are cultivated. A greenhouse is built of glass or plastic; it heats up because the sun's incoming electromagnetic radiation (particularly infrared light) heats up plants, soil and other things inside the building. Air warmed by the heat from hot interior surfaces and is retained in the building by the roof and wall.
The glass used for a greenhouse acts as a selective transmission media for different spectral frequencies, and its effect is to trap energy within the greenhouse, which heats up both the plants and the ground inside it. This warms the air near the ground, and this air is prevented from rising and flowing away. This can be demonstrated by opening a small window near the roof of a greenhouse: the temperature will drop considerably. Greenhouses thus work by trapping electromagnetic radiation, and then preventing convection.
Compare: greenhouse effect