Glasshouse and Greenhouse Effect

Whenever we enter an intact working glasshouse from outside in whatever season, we usually feel it is much warmer inside. People who are curious enough might want to figure out if there is some kind of furnace inside to heat the room up. However, their effort was destined to be fruitless in this case.

A glasshouse (or greenhouse) is a building built with glass, as its name indicates. It preserves energy from the sun during every sunny day hour and uses it to keep the room warm, without any other addition source of heat. So how does it work?

It works simply by setting up a “trap” for sunlight. We all know that light contains energy. When a light beam hits a surface, some of its photonic power is converted to heat. That is why it makes us feel warm during a cold chilly day. However, that does not explain why it makes the glasshouse warmer than the outside environment while both of them receive about the same amount of sunlight at the same time. The key to this puzzle lies in how light carries the energy.

To simplify the explanation, herein I will use an inaccurate but simpler illustration for the process (For the real business, please check here, in the entry of “Interaction of Light with Matter: Absorption and Emission of Light”). If you imagine the photon as a tennis ball that can bounce, then when the photon hits some other matters (e.g. ground), it loses some of its energy, which is converted into heat in the hit matter, and then bounces back with lower energy. The difference between a tennis ball and a photon is that your tennis ball will come back at a lower speed while the speed of light is constant in any given medium and what has changed in the return light is the frequency.

The glasses that is used to build a glasshouse act as a “fences” to block the tennis ball. It allows that light of high energy (visible light) to come into the glasshouse but prevent the light of low energy from going to either direction. Thus, when the light shines into the glasshouse, it is like you throw a tennis ball into a fenced court. The high-energy visible light can go into the glasshouse without much trouble. Nonetheless, after the light hit the objects (e.g. ground) inside the glasshouse, it loses energy to heat up the house and drop in its frequency to mostly infrared spectrum. Now, the infrared light is like a bounced-back tennis ball that you have just thrown into the fences. It cannot reach the same “height” as you firstly threw it because of the loss of energy. When the infrared tries to go out, it is blocked by the glass “fences” due to its low frequency. So it has to bounce back and hit (and heat) the ground again. This process repeats over and over so until the light’s energy is mostly exhausted. The temperature of the glasshouse is raised continuously during this process.

Carbon dioxide works exactly in the same mechanism, except this layer of “glass” is fluid and much thicker than any glass that we have ever seen. When the atmospheric CO2 concentration rises, less light energy can escape from the earth back to the space. It gradually makes the earth hotter and hotter, and finally change it from a shaded pavilion to a glasshouse.

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