Thermal radiation in veesible licht can be seen on this het metalwirk. Thermal energy would ideally be the amoont o heat required tae wairm the metal tae its temperatur, but this quantity is nae well-defined, as thare are mony ways tae obtain a gien body at a gien temperatur, an each o them mey require a different amoont o tot heat input. Thermal energy, unlik internal energy, is tharefore nae a state function.
