Incandescent light bulbs have been the most popular form of electrical lighting since Thomas Edison’s first practical ones 130 years ago. However, they have two major drawbacks:
- The way they use a hot filament to produce light wastes a lot of heat. More electrical energy is converted into heat than light.
- The filament does not get as hot as the Sun and so the light is not quite white but has a slightly yellow tinge. You see this in photographs taken at night with an incandescent bulb as the light source.
Gas discharge tubes, however, give off light when an electric current passes through the gas inside the tube. Even though they are more expensive to make and more complicated to run, they are far more energy efficient at producing light. The tubes are filled with small amounts of argon, neon and/or krypton and a tiny drop of mercury the size of a ballpoint pen tip. The mercury vaporises, and it is this vapour that is responsible for producing light. Mercury atoms absorb energy from the electric current that passes through the gas mixture and convert that energy to UV radiation with a wavelength of 254 nm. This is in the UVC part of the electromagnetic spectrum. Discharge tubes constructed from special quartz glass that allow the UVC to pass through are used to sterilise surfaces and disinfect water in air conditioning systems and sewage treatment plants.
Fluorescent lamps used for commercial and home lighting are essentially the same but use more ordinary glass that blocks UVC and UVB and they have a thin coating of phosphor powders on the inside of the glass. When the UV radiation emitted by the mercury atoms hit the phosphors, they fluoresce and produce both visible light and a little UVA that passes through the glass with very little energy lost as heat. Manufacturers use a mixture of phosphors that fluoresce with red, green and blue light so that together they appear white. Two main blends of phosphors are used – one that simulates sunlight and produces a ‘cool’ white and the other that simulates incandescent light and is called ‘warm’ white.
Compact fluorescent lamps (CFLs) have been developed to take the place of incandescent light bulbs. They use approximately one-fifth the electrical energy to produce the same amount of light and last up to eight times longer. The new technology of LED light bulbs are even more energy efficient, converting up to 40% of the electrical energy into visible light.
Other special-purpose fluorescent tubes are made to produce various amounts of UVB and UVA using combinations of types of glass and phosphors:
- UV lamps are used in manufacturing to cure some resins, glues and inks.
- Sunbed lamps emit large amounts of both UVB and UVA.
- Tubes used for aquariums and indoor hydroponic gardens emit UVA as well as visible light.
- Black lights are designed to give off UVA and very little visible light. They are used for special displays and at parties.
Light emitting diodes (LEDs) that are even more energy efficient have been made to shine with a faint purple light and UVA. With improvements, these could replace black lights in coming years.
To make it difficult for addicts to inject drugs when in public toilets, some authorities have used lighting comprising UVA and visible blue. The lack of red in the lighting makes it very hard for the addicts to see a red vein under their skin and so puts them off using the toilets for injecting drugs.
Follow up with one of these activites to help extend your students learning:
- Rock fluorescence – use UVA and UVB light sources to identify certain kinds of minerals in rocks.
- Write a secret message – use a UV light-emitting diode pen to write secret messages, then investigate a range of objects to see if or how they fluoresce.
- Make a printed circuit board – make a printed circuit board photographically by shining UVA light through a negative artwork mask to cure and harden a thin coating of UV-sensitive material that covers the copper.