What is the principle of electric tube light?

Answer 1

The way an electric tube, or fluorescent, light works is really very interesting. Most "regular" light bulbs are incandescent. Their source of light is the glow of electricity passing through a very thin wire known as a filament that is suspended in a vacuum. It glows with a "warm" color in the yellow/orange range. With a fluorescent tube there are two filaments, but they do not directly provide the light source, and a Mercury vapor instead of a vacuum.

On each end of a long glass tube are coils using electricity regulated by a ballast. The tube is filled with mercury vapor and phosphor. The electricity passing through the filaments excites the mercury vapor causing it to glow with ultraviolet (black) light. Ultraviolet light is not visible, but it does cause cause certain chemicals and colors to glow or fluoresce. If you take laundry detergent with phosphorous in it, for example, and place it under an ultraviolet (black) light, the phosphorus glows cold white. So by adding Phosphorus to the tube of Mercury vapor it will also glow when the vapor is excited electrically.

The result is a lot of light for a much lower amount of electricity, and excess heat. The color temperature is very close to that of daylight but a little cooler, in the blue/green range. That makes growing plants under fluorescent light more effective also. The negatives are that most "tube lights" can not be dimmed with a wall dimmer-switch, and the ballasts, that controls electrical flow, do have a tendency to burn out or fail after a time.