How does a glass thermos keep hot coffee warm?
I believe it all has to do with the way heat is lost. In the case of hot coffee in a thermos, the bottle surface is bright silver which is reflective to both light and heat. The major part of the radiation which would occur without a thermos, is reflected back into the coffee and therefore does not lose that heat. Heat moves by 3 ways: conduction, radiation and convection. Conduction is the movement through something, liquid gas or solid. The glass in the thermos is a poor conductor and so coffee does not cool down because of that (much). On the other hand if you took a coffee cup filled with hot coffee and placed it in a sink of water (up to the top but not covering it), the coffee would rapidly cool down due to heat flowing to the water via conduction. Radiation is the 2nd method important for cooling coffee, or keeping it hot. Hot coffee in a cup will warm the cup itself, and the cup will radiate its heat into the room. In a thermos the reflective nature of the glass bulb basically
