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What is "cold light"? Why can some light shine without being heated?
Most of the light we normally see comes from thermal radiation from high-temperature objects—for example, the sun or a glowing incandescent filament. However, there is a type of light that shines almost independently of high temperatures. This type of light is called luminescence.
I. Definition of Luminescence
When the intensity of light emitted by an object exceeds the brightness it would have at the same temperature according to the laws of thermal radiation, we call this light "luminescence."
In other words, it is not generated by high temperatures, but rather by other forms of energy.
II. Principles of Luminescence
The excitation energy for luminescence differs from that of thermal energy.
It may come from:
Electron bombardment (e.g., cathode ray tubes)
Chemical reactions (e.g., fireflies)
Electric fields or light (e.g., fluorescent lamps, LEDs)
Because this type of luminescence does not rely on high temperatures, it is not subject to thermodynamic constraints, and luminescent materials can operate stably near room temperature.
III. Two Main Forms of Luminescence
1. Fluorescence
Once the excitation energy disappears, the light disappears immediately, lasting only a very short time. Examples: fluorescent lamps, phosphors.
2. Phosphorescence
Luminescence continues even after excitation ceases, slowly decaying. Examples: luminous watches, luminous stickers.
IV. Examples of Cold Light in Everyday Life
LED lights
Fluorescent lamps
Neon lights
Firefly bioluminescence
Luminous signs
These luminous phenomena may appear "cold," but they all stem from a sophisticated process of energy conversion, a marvelous fusion of nature and human technology.
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