Incandescence
Definition and meaning of Incandescence in chemistry.
Incandescence is the emission of visible light by a material because it has been heated to a high temperature, not because of a chemical reaction or absorbed radiation.
In more detail
As a substance is heated, its atoms and electrons gain thermal energy and re-radiate it as a continuous spectrum of electromagnetic waves, a process described by blackbody radiation theory (Planck's law). At low temperatures this radiation lies in the infrared, but once a solid reaches roughly 525 degrees Celsius (about 798 K), enough of the spectrum shifts into the visible range for a dull red glow to appear, the classic "red hot" effect. As temperature rises further, the peak emission wavelength shortens according to Wien's displacement law, so the glow progresses from red to orange, yellow, and finally white as more of the visible spectrum is emitted.
Key facts
| Field | Physical Chemistry |
|---|---|
| Mechanism | Thermal (blackbody) radiation, not a chemical reaction |
| Visible glow onset | ~525 °C (798 K) |
| Governing law | Wien's displacement law / Planck's radiation law |
A tungsten filament in an incandescent light bulb is heated by electrical resistance to about 2500 to 3000 degrees Celsius, causing it to glow white hot and emit visible light through incandescence.
Frequently asked questions
How does incandescence differ from fluorescence or chemiluminescence?
Incandescence produces light purely from an object's heat (thermal radiation), whereas fluorescence requires absorbing light of one wavelength and re-emitting it at another, and chemiluminescence produces light from a chemical reaction, both largely independent of temperature.
Why does a heated metal change color as it gets hotter?
As temperature rises, the wavelength of peak thermal emission shortens according to Wien's displacement law, shifting the visible glow from dull red to orange, yellow, and eventually white.