What’s the difference between a compact fluorescent and a regular bulb? (#NSTA11)

by Stephanie Chasteen on March 15, 2011

I wanted to blog more from the National Science Teachers Association (NSTA) meeting last week but (a) I hardly had any time to go to sessions since I was busily hawking the best software money can’t buy (the free PhET simulations) and other busy-making things, and (b) there was no wifi at the hotel or the conference.  I usually only manage to liveblog when I can, well, actually blog live.

But I did manage to make it to a few sessions and I wanted to share this completely charming demonstration of the different light intensities from two different types of bulb — a regular incandescent bulb and a compact fluorescent.

Take a piece of paper.  Put a smudge of mineral oil on it.  It turns somewhat translucent where the mineral oil is.  If you hold it up to a light bulb, the smudged area looks bright.

Now set up two light bulbs, straight across from each other — one regular bulb and one compact fluorescent.  Both bulbs should be the same power — 15 W.  That’s the amount of energy per second that each bulb is drawing from the electrical outlet.

Put the mineral-oil smudged paper between them and stand behind one of the bulbs.  Does the smudged area look lighter or darker than the paper around it?  Do you agree with another person standing behind the other bulb?

Image from Lawrence Hall of Science

The person looking through the paper at the regular bulb should see the smudge looking darker than the surrounding paper.  But the person looking through the paper at the compact fluorescent sees it looking brighter. This can lead to some great investigations.  You can move the paper around until both people agree that the illumination of the paper is equal to the illumination of the smudge.   You can measure the distance of the paper to the bulbs at that point.  You can bring out a light meter to measure the illumination at these various points.

What’s going on?  Because the oil-smudged part of the paper is translucent, the light mostly passes through it.  But the surrounding paper is opaque, so light reflects from it.  Thus, the brightness of the paper indicates the amount of light reflecting to your eye from the bulb that you are standing near (say, Bulb A).  Whereas the brightness of the oil smudge indicates the amount of light reaching your eye from the other bulb (say, Bulb B).  Because the light intensity is greater for a compact fluorescent of the same wattage, when the paper is midway between the two light bulbs, the compact fluorescent “wins.”  If you move the paper to a place where the illumination is equal, it will be much closer to the regular bulb.  How much closer?  Light intensity falls off as one over the square of the distance, so you can do a little math.

You can see a PDF of this activity here.

This activity is part of a set of hands-on activities developed by the Lawrence Hall of Science Global Systems Science Curriculum. You can download the curriculum and the teachers’ guides for free from their website.

The really cool thing about this program, though, is that they’re creating Professional Learning Communities (PLC’s) for high school teachers around climate change — the Lifelines for High School Climate Change Education project.  Anyone can join one of these PLC’s, they’re in several states.  I’m considering starting one for Colorado.  Check it out and fill out the online application!

Image by Alphax on Wikimedia.

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