Greenhouses trap heat primarily because their glass (or transparent plastic) walls allow sunlight to enter but prevent much of the infrared radiation (heat) from escaping.
Here's a breakdown of the process:
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Sunlight Enters: Sunlight, which consists of various wavelengths of electromagnetic radiation, including visible light, easily passes through the glass or plastic of the greenhouse.
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Absorption and Re-emission: When sunlight strikes the plants, soil, and other objects inside the greenhouse, these surfaces absorb the light and warm up. These warm surfaces then re-emit energy as infrared radiation, which has longer wavelengths.
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Infrared Radiation is Trapped: While glass is transparent to visible light, it is less transparent to infrared radiation. The glass absorbs and re-emits some of the infrared radiation, but a significant portion is reflected back into the greenhouse, preventing it from escaping. This reflected energy contributes to the warming of the air and contents inside the greenhouse.
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Convection is Reduced: The enclosed structure of the greenhouse also reduces heat loss through convection. The walls block wind and air currents, minimizing the amount of warm air that can escape and be replaced by cooler air from the outside.
In essence, the greenhouse acts as a one-way valve for solar energy. It allows sunlight in but restricts the escape of heat, leading to a warmer environment inside compared to the outside. This is why even on cold days, greenhouses can maintain a suitable temperature for plant growth.
