Why Do Light Bulbs Have Glass Bulbs?
The glass bulb protects the light’s filament. Inside a light bulb, electricity passes through a thin wire called a filament. The electricity makes the filament so hot that it glows with a bright white light.
If surrounded by air, the heated filament would soon burn up. To prevent this, the filament is placed inside a glass bulb and the air is drawn out of the bulb. The bulb is then sealed and a metal cap is cemented to its base. Before the light bulb is sealed up, its inner surface is sprayed with a white coat to soften the glare of the light.
Most light bulbs have either clear or coated glass. The coated glass bulbs have a white powdery substance on the inside called kaolin. Kaolin, or kaolinite, is white, chalky clay in a very fine powder form, which is blown in and electro statically deposited on the interior of the bulb.
It diffuses the light emitted from the filament, producing a gentler and evenly distributed light. Manufacturers may add pigments to the kaolin to adjust the characteristics of the final light emitted from the bulb. Kaolin diffused bulbs are used extensively in interior lighting because of their comparatively gentle light.
Other kinds of colored bulbs are also made, including the various colors used for “party bulbs”, Christmas tree lights and other decorative lighting. These are created by coloring the glass with a dopant; which is often a metal like cobalt (blue) or chromium (green). Neodymium-containing glass is sometimes used to provide a more natural-appearing light.
Many arrangements of electrical contacts are used. Large lamps may have a screw base (one or more contacts at the tip, one at the shell) or a bayonet base (one or more contacts on the base, shell used as a contact or used only as a mechanical support). Some tubular lamps have an electrical contact at either end.
Miniature lamps may have a wedge base and wire contacts, and some automotive and special purpose lamps have screw terminals for connection to wires. Contacts in the lamp socket allow the electric current to pass through the base to the filament. Power ratings for incandescent light bulbs range from about 0.1 watt to about 10,000 watts.
The glass bulb of a general service lamp can reach temperatures between 200 and 260 °C (392 and 500 °F). Lamps intended for high power operation or used for heating purposes will have envelopes made of hard glass or fused quartz.