人类老早就发现,发光的物体——光源,一般都是很热的。太阳就是很热的。太阳是光同时也是热的无尽泉源。它表面温度高到六千度。绝大部分的人工光源也是很热的。篝火、油灯、白炽电灯和弧光灯等,温度也高到一、二千度或三、四千度。然而,却不是一切光源都是很热的。只要稍稍留意一下,就会发现:也有不热的光源。例如,萤火虫(图1)会发光,但是它一点也不热;能在夜间闪闪发光的夜光表,也不是热的;还有,霓虹灯和日光灯也不算很热;……。如果说,从太阳之类很热的光源发出的光,可以叫做“热光”的话,那麽,从萤火虫之类 Humans have long discovered that glowing objects, light sources, are generally hot. The sun is hot. The sun is light and it is also an endless source of heat. Its surface temperature is as high as six thousand degrees. Most of the artificial light sources are also very hot. Flames, oil lamps, incandescent lamps, arc lamps, etc., have temperatures as high as one or two thousand degrees or three or four thousand degrees. However, not all light sources are hot. As long as you look a little, you will find that there are also non-hot light sources. For example, a firefly (figure 1) will glow, but it is not hot at all; a luminous watch that glows at night is not hot; and neon and fluorescent lamps are not too hot. If we say that light from a very hot light source such as the sun can be called “hot light,” then from fireflies and the like