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Astronomy/electromagnetic radiation


QUESTION: This may be more of a physics question, but here is my question:

Is all electromagnetic radiation ultimately converted to heat energy? As a specific example, a 100 watt incandescent light bulb in a home emits light and heat.  Does the light portion also end up as heat, so that the bulb is adding 100 watts of heat to the house?

Thanks for your help, trying to understand heat balance for our house.


I hope my salutation isn't a description of your relationship with your utilities company--and that you have a HIGH BILL!

Incandescent bulbs are a perfect example of a not-very-efficient lighting source, for exactly the reason you point out. An incandescent bulb actually emits a lot of heat in addition to the light it emits.  Compare that to a much more energy efficient fluorescent bulb, that generates almost no heat--and still emits the same amount of light as an incandescent bulb.  

incandescent bulbs work by transforming electricity into heat---the light comes from a small filament that is heated until it is white hot--and then into light.  That's not efficient.  Fluorescent bulbs and tubes use that electricity to energize a gas until it glows without heating it much, and thus they are much more energy efficient.  And they don't heat your house up at all!

Hope that helps

Paul Wagner

---------- FOLLOW-UP ----------

QUESTION: Thanks Paul, appreciate the humor!

Yes your answer helps. But from a technical/physics analysis, does the light emitted also become heat after the light hits surfaces and (maybe) heats the surfaces?

I mentioned heating of a house interior, but I'm also curious from the scientific point of view.


Hi Bill

Light (lumens) itself isn't a very good measure of whether electromagnetic radiation will contain heat.  Here's a link to a NASA article about the subject.  Note that everything from radio waves to Gamma rays are part of the electromagnetic spectrum. Infrared radiation will radiate heat quite well, but it is also invisible in the spectrum that can be seen by the human eye. (Other animals can "see" infrared.)

So you need to look at each light source to see how much of each type of EM radiation is being emitted, and then calculate how much of that is reaching the walls.  For an incandescent bulb, that can actually be fairly high---just ask anyone who has been "bathed" in the focused lights of a well-lit theater stage.  But if you were to light that same stage with equally bright fluorescent lighting, the heat received by those on the stage would be almost negligible.  

Of course, they would look very different in that kind of light--which is why it isn't used in theaters.  Does that help?

Paul Wagner


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Paul Wagner


Astronomy and telescope making. Have made at least seven telescopes, both refractors and reflectors, and have spent 30 years looking at the nighttime sky.

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