Radiation
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Thermal radiation is a process where an object radiates thermal energy from its surface in the form of electromagnetic waves, usually infrared radiation. Thermal radiation is caused by electromagnetic waves including infrared and light and they can travel through empty space.
As objects absorb these waves they absorb energy and get hotter as a result. Warm objects normally give off infrared but if they become really hot they can start emitting light. That is why a light bulb glows when it heats up. Basically, thermal radiation can be absorbed, reflected or transmitted through a body.
Absorbers, Emitters & Reflectors
Some materials are good at absorbing and emitting thermal radiation while others are good at reflecting radiation. Examples of good absorbers and emitters are matt black materials. White and silvery surfaces are bad absorbers because they reflect away most of thermal radiation. However, bad absorbers are good reflectors and likewise good absorbers are poor reflectors of thermal radiation.
In summary:
Matt Black Materials = good absorbers/emitters + poor reflectors
White/Silvery Materials = poor absorbers/emitters + good reflectors
The Vacuum Flask
A Vacuum flask has many features to reduce not only thermal radiation but also conduction and convection. These features include:
1. An insulated stopper which reduces conduction and convection.
2. A container with two walls with a vacuum gap to stop conduction and convection.
3. Walls that have silvery surfaces to reduce thermal radiation.
These features enable the flask to keep drinks warm or cold for a long time.