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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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).

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.

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 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.