How Is Light Produced?
You already learned that a substance called ether is found in all substances, filling the spaces between the molecules. When the molecules are made to vibrate, the ether naturally also vibrates. As soon as the vibrations become sufficiently rapid, they produce the sensation of light. These vibrations also produce heat. In heated bodies the molecules are always found to be in vibration, and a body may become so hot that it gives off light. We notice this when iron becomes red hot. Heat and light are found together in bodies in many instances. In fact, most of the light we have comes from bodies which are hot. The sun is so hot, that it is surrounded by the gases of many substances that exist as solids on earth.
We have some bodies which produce light which is not accompanied by much heat. The glow-worm, or firefly, seems to make light with little or no heat; but we do not yet know how this is done. Almost all sources of artificial light require that heat be produced before light obtained. Only such vibrations of the ether which are sufficiently rapid produce enough light to enable us to see. For this reason, a piece of red hot iron, which is made luminous by heat and whose particles vibrate less rapidly produce little light.