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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 16 January, 2003 Next Week's Question
Halogen lights burn longer because of iodine, bromine
Question
Why are some lights called Halogen Lights? Do they contain elements from the Halogen group?

Question
Halogen light bulbs do contain small quantities of elements from the halogen group, most frequently iodine or bromine. In fact, the presence of these elements is what allows halogen bulbs to be brighter and to last longer than regular bulbs.

In a light bulb, electricity passes through the tungsten filament, causing it to heat up and give off light. The filament becomes so hot that the tungsten metal slowly evaporates, turning to vapor and then solidifying as a thin layer on the inside of the glass bulb. Eventually, the filament weakens and breaks, and the bulb "burns out." A brighter bulb has a hotter filament, which causes faster evaporation of the tungsten. There is a limit to how bright a regular light bulb can be because the filament in a very bright bulb would thin and break too quickly to be practical.

In a halogen bulb, light is produced in the same way, and tungsten metal still evaporates from the hot filament. However, the evaporated tungsten reacts with the halogen in the bulb to form a compound called a tungsten halide. This compound forms at the relatively low temperature near the glass wall and breaks apart at the high temperature of the filament, redepositing tungsten metal on the filament. The halogen serves to transport tungsten back to the filament. A halogen bulb can be brighter and hotter than a regular bulb and can last longer because reaction with the halogen repairs the filament!

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