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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Heat gets in as light.
Glass is transparent to light.
The light hits the stuff inside: soil, plants etc and heat them up.
The ground and the plants inside the glasshouse are in contact with the air inside the glasshouse.
The air heats up inside the glasshouse.
The air is trapped in the glasshouse and can’t get out easily.
A bit escapes and a bit of the heat gets transferred to the glass panes and from there to the outside air, but more heat is trapped than would be if there was no glass in the way.
TL;DR: The heat gets in a different way then it would get out and the glass allows the way in (light) but not the way out (air movement).
Your premise is wrong. Glass generally blocks heat in either direction, but is transparent to visible light and ultraviolet. The absorbed light that passed through the glass may re-radiate as heat, and so get stuck inside.
Glass also prevents warm air from leaving the structure.
The heat you feel when standing out in the sun is mostly from electromagnetic radiation from infrared light. This is why, on hot days, it can be significantly cooler when you are in the shade. Infrared light is really good at passing through glass. The infrared is then absorbed by objects within the greenhouse, which in turn emits the energy back out as heat through convection into the air in the greenhouse. Once the heat energy is in the air, the greenhouse’s insulation traps that air inside of it.
The same reason the inside of a car gets hot in the sun. Light comes in through the glass windows and is absorbed by the upholstery and other interior stuff, which emits a comparatively negligible amount of radiation back out of the car.
Greenhouses work similarly (albeit a bit more deliberate). Light enters through the glass and is absorbed by plants and other stuff sitting in the greenhouse. That energy cannot escape the glass by convection and very little gets out by conduction.
Heat takes many forms. Heat coming from the sun is largely visible light, which of course passes through glass. Heat trying to escape is either infrared light (which struggles to go through glass) or hot air trying to escape. The latter, obviously, cannot pass through glass.
Heat from the sun is shortwave, ultraviolet light.
As it goes through the glass it is turned into longave, infrared light.
The infrared light can’t penetrate the glass to get out
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.
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).
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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