Solar heat transmissivity trough glass...

[Externet]

Hi.

I learned somewhere that sun radiation (heat + light) passes trough glass as in a window, but the heat does not escape again towards the outside trough the glass, bouncing back and getting trapped indoors as in a greenhouse.

 

I have trouble understanding the effect; does it mean an item enclosed in a glass jar indoors will not get warm as the heat will not pass trough glass again ? (except the amount of heat transferred by convection from room air to jar)

[CaptainPanic]

Glass is transparent for visible light. As you can see in a window, light goes straight through it.

 

But glass is not transparent for heat (infrared radiation). Instead, it reflects and absorbs heat.

 

But inside the house, most heating is done through convection, not radiation. It's a different type of heat transport, most importantly by the circulation of air. So, your jar of pickles will probably be the same temperature as the rest of the room... especially if you give it some time after you bring it in from the shop.

[swansont]

Hi.

I learned somewhere that sun radiation (heat + light) passes trough glass as in a window, but the heat does not escape again towards the outside trough the glass, bouncing back and getting trapped indoors as in a greenhouse.

 

I have trouble understanding the effect; does it mean an item enclosed in a glass jar indoors will not get warm as the heat will not pass trough glass again ? (except the amount of heat transferred by convection from room air to jar)

 

A good example is a car left out in the sun. It can get much warmer than the ambient temperature because you have largely removed conduction and convection as methods of heat transfer to the surroundings (other than the car itself). The incoming radiation is somewhat shorter than 1 micron and passes through the glass. The warm interior radiates at around 10 microns, but this is trapped by the glass — reflected or absorbed. The only other removal method is from heating up the car frame itself, which can then conduct, convect and radiate. Interior steady-state is achieved at a higher temperature than ambient.

[J.C.MacSwell]

Hi.

I learned somewhere that sun radiation (heat + light) passes trough glass as in a window, but the heat does not escape again towards the outside trough the glass, bouncing back and getting trapped indoors as in a greenhouse.

 

I have trouble understanding the effect; does it mean an item enclosed in a glass jar indoors will not get warm as the heat will not pass trough glass again ? (except the amount of heat transferred by convection from room air to jar)

 

So your item will in fact heat up, just as you would feel the heat of the sun on your face looking through a pane of glass...and when it does it will radiate, but have that radiation reflected back.

Edited May 1, 2012 by J.C.MacSwell