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Q: Magnification of Low-Level Light ( No Answer,   6 Comments )
Subject: Magnification of Low-Level Light
Category: Science > Instruments and Methods
Asked by: ocoeeriver-ga
List Price: $2.00
Posted: 09 Mar 2006 12:33 PST
Expires: 08 Apr 2006 13:33 PDT
Question ID: 705451
We know that one can use a magnifying glass to focus sunlight and
start a fire.  But can one use a magnifying glass (or maybe even
multiple magnifying glasses) to take, say, LED light or a light bulb's
glow and start a fire?  That is, does the heat of the beam of light
depend on the source or on the magnification being used?  Thanks.
There is no answer at this time.

Subject: Re: Magnification of Low-Level Light
From: myoarin-ga on 09 Mar 2006 14:15 PST
It isn't the visible light but rather the heat from the sun that is
concentrated by a magnifying glass to start a fire.  Not many  - if
any -  man-made lights produce enough heat to do this.  LEDs are not
hot and never will.
Subject: Re: Magnification of Low-Level Light
From: kottekoe-ga on 09 Mar 2006 21:51 PST
Myoarin, that's not true. Heat is just moving particles, you can't
focus it with a lens. The hot surface of the sun produces light. The
magnifying glass concentrates the light. The peak of the spectrum is
in visible wavelenghts. When light of any wavelength is absorbed, it
heats up the absorber. The sun's radiation is close to an ideal
thermal spectrum (so called Black Body radiation) that any black
object of the same surface temperature would produce. The best you can
do with a lens is to heat something up to the same temperature as the
sun's surface (ignoring small corrections related to the index of
refraction). This is an upper limit. Any practical lens will do worse.
With an incandescent light bulb, you could in principle focus the
filament to produce a temperature equal to the filament's temperature.
Not as hot as the sun, but hot enough to burn paper.

An LED or laser is a different story since the spectrum is highly
non-thermal. In general the light can be concentrated much better than
that from an incandescent source and, with a bright enough LED or
laser, you can start something on fire.
Subject: Re: Magnification of Low-Level Light
From: myoarin-ga on 10 Mar 2006 02:16 PST
Kottekoe is right, of course.  Sorry. :-)
Subject: OK, then, that being the case...
From: ocoeeriver-ga on 21 Mar 2006 09:33 PST
It would seem to me that we could magnify the light from lightbulbs to
such a degree to create turn steam turbines, etc.

I mean, if you have ONE lightbulb and magnify it, then magnify THAT,
and so on, wouldn't at some point you have enough heat energy to
create steam...and perhaps enough power from that to MORE than power
the light bulb?

Yes, that smacks of perpetual motion.  But, hey, I'm just wondering....
Subject: Re: Magnification of Low-Level Light
From: myoarin-ga on 21 Mar 2006 16:51 PST
You are right that it sounds like perpetual motion.  It won't work. 
Creating light with an incandescent bulb is inefficient.  There are
better attempts at perpetual motion.
Subject: Re: Magnification of Low-Level Light
From: kottekoe-ga on 21 Mar 2006 20:02 PST
Myoarin is correct. You can work this all out with optical theory and
find that you cannot focus the light to produce a temperature greater
than that of the filament, that it does no good to focus more than
once, and so on. But it is easier to just invoke conservation of
energy and skip the detailed analysis. All the scientific and
engineering laws obey the principle of conservation of energy. That is
why we know without further analysis that a perpetual motion machine
can never work if there is ANY source of energy loss.

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