Editorial--Lights and Inquiry

Submitted by Shawn Reeves

2007-05-13 13:15:17

Based on ideas from Stephen Cremer, Alison Nash, Osram Sylvania, and many students.
Consumers today are asking questions about lights questions without textbook-answers: What is a fluorescent light? Should I leave lights on if I’m not using them only for five minutes? Aren’t these lights flickering? How much energy going into this lamp comes out as visible light? How does an occupancy-detector work?
Lighting-technology has changed appreciably in the past few years, so answers may be different than when you last learned about lights. Fortunately, there is a pedagogy that works very well on topics about which the teacher knows little more than the student: Inquiry. (Inquiry also works well in all subjects, but novel topics force the teacher into a mode of less predictability, where there are higher risks but greater rewards.)
To use inquiry to teach, we must inculcate our students with questions that require primary or secondary research, synthesizing pre-existing science with thoughtful analysis, hopefully leading to inferences that give satisfying answers to the initial questions, and possibly leading to more questions.
In the spirit of inquiry, I would like to lead you just towards some ideas and leave you to further research. I will introduce a few topics of lighting engineering and science.
Try to get your hands on a flicker-checker, a spinning top that helps determine whether lights use magnetic ballasts, which switch fluorescent lights on and off at a humanly perceptible frequency, or they use electronic ballasts, which switch must faster. Can you use the top to estimate the frequency of the flicker? What is the highest frequency that a student can perceive?
The tops are available from the National Energy Education Development Project:
The title "Flicker Checker" is a registered trademark of Osram Sylvania.
The 15-Minute Myth
Consider this paragraph from
"You can save by turning off incandescent lights when you leave the room. If you use fluorescent lighting, however, turn them off only if you will be gone longer than 15 minutes because fluorescent lights use as much energy in starting as they use during 15 minutes of operation, so it's not worthwhile to turn them off for brief periods."
Could a light that takes 1 second to turn on fully (as fluorescents these days do) use as much energy in that second as it would in 15 minutes of normal operation? I figure that 15 minutes (900 seconds) of 40 Watts is 36000 Joules. 36000 Joules in 1 second is 36000 Watts, enough to light a supermarket, and enough to blow any circuit breaker you might have in your house. Is my analysis too simple, thus my inference too dismissive? Try measuring the energy use of lights, as you turn them on and off, with a data-logging (and graphing) watt-meter, which you can buy from EnergyTeachers.org or from several other sources.
How much of the energy going into a light as electrical energy comes out as visible light? Also, when talking about the portion of light that isn't useful for seeing, infrared light is often called "heat." Doesn't visible light, as well as all any other wavelength, also transmit energy that can change the temperature of materials?
You may start your inquiry by browsing a helpful article in Wikipedia about efficacy:
How do the lights in your room compare to lights in Wikipedia’s tables? What tools would you need to measure your lights' efficacy? Which type of lamp, LED, fluorescent, incandescent, spark gap, or other, has the highest theoretical limit of efficacy?

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