why is it that heat penetrating glass is a one way thing? Is in a greenhouse, heat will penetrate the glass and get trapped inside the greenhouse.
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A couple of the answers as of the time of this posting are incorrect. Glass is transparent to visible light, which is the portion of the electromagnetic spectrum where a lot of the sun’s energy is emitted, but blocks infrared light.
The sun emits visible light because of its surface temperature of thousands of degrees (no matter which way you measure it). This visible light carries energy into the greenhouse as the electrons in the glass don’t have the freedom to interact with light of this energy. Plants in the greenhouse absorb the visible light and warm up. Everything that is at least somewhat warm will glow with some kind of light, and for the temperatures in the greenhouse the things glow with infrared light.
Infrared light has less energy per photon than visible light, and the electrons in the glass are able to interact with infrared light. They absorb the light, which keeps the energy from easily leaving the greenhouse. The glass is thus a one way valve of sorts for the light energy, letting it in when it’s the visible light from the sun and blocking it when it’s the infrared light from the interior of the greenhouse. Carbon dioxide does the same thing, which is why the Earth is warming. If you could see infrared light, glass would be opaque and car exhaust would be dark and smoky looking.
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