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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Everything in the universe “glows,” meaning it sends out particles called photons.
Photons have different energy levels. Our eyes can see photons within a certain narrow range of energy levels.
Hotter things send out more photons with higher energy levels. So we can directly see things glowing if they’re very hot. We can also build machines to see photons with lower energy levels. (Night vision goggles are machines that can distinguish the glow of things that are a little bit hotter than their surroundings, even if they aren’t hot enough for us to see the glow directly.)
When photons run into things, they make those things hot. (Photons with extremely high energy levels can also do other things, like knocking out parts of atoms, which has a tiny chance of causing cancer if it happens inside a living creature. Which is why we get X-rays only when necessary for a doctor who needs to see a problem inside us, and not just for fun whenever we want.)
Heat travels by three different ways. In order from greatest to least, they are: Self-mixing of gases or liquids (convection), contact (conduction), and photons (radiation).
So, greenhouses.
Radiation: Photons from the Sun have no trouble traveling through glass (that’s why you can see through it) [1]. The Sun is really hot so it has a powerful glow and sends out a huge number of photons, so the greenhouse receives a lot of energy by radiation. The greenhouse is just a normal temperature that’s comfortable to us humans, so it sends out very little energy by photons.
Convection: Inside the greenhouse the air can mix itself freely, but it’s blocked by the glass and can’t mix easily with outside air. So the greenhouse can’t really lose or gain heat from outside by convection since the glass mostly blocks off the air inside from the air outside.
Conduction: A greenhouse can lose heat by conduction. But conduction happens based on the material, contact area and temperature difference. Glass is an…okay conductor of heat. Conduction does eventually stop a greenhouse from just getting hotter and hotter until it melts or catches fire, but it just doesn’t work fast enough to dump the heat from all those photons until you have a temperature difference of maybe a couple tens of degrees. Which is just about right for growing plants in all seasons.
So the thing that’s different between the inside and outside is that the Sun is outside and it’s really hot. There’s nothing comparably hot on the inside.
[1] Actually, whether a photon can travel through a thing or gets absorbed depends on its energy level (as well as the material the thing is made out of). Radio, TV, cell phones and wifi communicate by sending very low energy photons that can travel through most materials including the walls of buildings.
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