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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It’s not one way.
Greenhouses are made mostly of glass. Glass is transparent, meaning visible light goes through it. Light isn’t hot or cold, because those words describe the temperature of materials. Light is going in, not heat.
The light that goes in slams into whatever is inside the greenhouse: plants, dirt, air, water, tools, etc. Some of that light bounces off, some of the energy is absorbed, which warms the material. Some of that light bounces off while loosing energy to the material so it the reflected light has lower energy, which changes its color. The glass isn’t transparent to all colors, particularly lower energy light called “infrared” (which human eyes can’t see). That light is trapped inside the greenhouse and bounces around until it loses its energy slamming into things in the greenhouse and warming them up.
The greenhouse is converting energy in the form of light into energy in the form of heat.
The stuff inside in the greenhouse warms the actual glass itself, and that glass will allow heat to pass from the warmer side to the hotter side. It’s not a thermos. However, speed that the stuff inside warms the glass, and the glass passes it through, is not very fast by comparison, so the heat can back up.
You see the exact same effect with certain gases like methane and carbon dioxide. They do the same thing as the glass panels, allowing light through to warm stuff, but preventing the low frequency light out, effectively trapping the energy and warming everything on the side opposite the light source. This was first described and accurately measured in the 19th century, but wasn’t given its modern name “the greenhouse effect” until a 1901 (later popularized in a 1912 Popular Mechanics article on the effects of human industry on climate).
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