lenses and the 2nd law

[RichD]

This may be a dumb question, but anyway....

A lens focuses energy, does it not? That's its
purpose. So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

Does this not violate the 2nd law of thermodynamics?
The natural evolution is the radiation spreads,
becoming more disorderly and lower temperature.
The lens reverse this process, concentrating the
energy, with temperature rise.

And the lens is a passive device, so no exogenous
power source to do the work.

Ot it could be a parabolic mirror, same thing.

Explanation?

--
Rich

 

[Sam Wormley]

On 1/23/11 6:11 PM, RichD wrote:

This may be a dumb question, but anyway....

A lens focuses energy, does it not? That's its
purpose. So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

Does this not violate the 2nd law of thermodynamics?
The natural evolution is the radiation spreads,
becoming more disorderly and lower temperature.
The lens reverse this process, concentrating the
energy, with temperature rise.

Focus the sun's energy coming through the earth's
atmosphere. The achievable temperature at the focal
point is is always less than that of the source, in
this case, the surface of the sun.

 

[Phil Allison]

"RichD"

This may be a dumb question, but anyway....

A lens focuses energy, does it not? That's its
purpose. So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

Does this not violate the 2nd law of thermodynamics?
The natural evolution is the radiation spreads,
becoming more disorderly and lower temperature.
The lens reverse this process, concentrating the
energy, with temperature rise.

And the lens is a passive device, so no exogenous
power source to do the work.

Ot it could be a parabolic mirror, same thing.

Explanation?

** Take a look at Lenz's Law.


..... Phil

 

[Timo Nieminen]

On Jan 24, 10:11 am, RichD <r_delaney2...@yahoo.com> wrote:

This may be a dumb question, but anyway....

A lens focuses energy, does it not?  That's its
purpose.   So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

Yes. (When talking waves, "dissipating" usually means "being
absorbed", that energy is being lost from the wave, and isn't usually
used for the drop in intensity due to a wave spreading.)

Does this not violate the 2nd law of thermodynamics?

No, because ...

The natural evolution is the radiation spreads,
becoming more disorderly and lower temperature.

.... the spreading of the radiation from a source doesn't mean that the
temperature of that radiation falls. I also wouldn't call it more
"disorderly" as it spreads. (You need to be careful about
"disorderly". Yes, you can give the 2nd law in terms of "disorder",
but that is for a specific technical meaning of "disorder".)

The lens reverse this process, concentrating the
energy, with temperature rise.

The converse of the above is that there is no temperature rise of the
radiation.

Consider a radiating object O of temperature To, enclosed in a box of
lower temperature Tc. If we have another object Sin the box, it will
reach a temperature between Tc and To. If S is a long way away from
the radiating object O, the temperature will be close to Tc. If S is
close to O, its temperature will be closer to To.

Object S doesn't know how close it is to O; it doesn't directly
interact with O. It only interacts with the radiation from O and from
the surrounding box. Think about a bunch of rays spreading out from S.
These are the lines along which S will interact with its surroundings
via radiation. What matters is whether the rays will hit O or the
surroundings. What a lens (or paraboic mirror) will do (if in the
correct position) is deflect these rays so more of them hit O than
without the lens. The effect is the same as moving S closer to O. So
the temperature of S gets closer to that of O. The best you can do is
completely surround S by radiation from O (e.g., by surrounding both S
and O with a spheroidal mirror), in which case S looks like it's
surrounded by O. Same effect as being in an oven at To; the
temperature of S will be To at equilibrium.

The energy flow is only from hot to cold, so the laws of
thermodynamics are perfectly happy, with no work required.

 

[Salmon Egg]

In article
<3a96de31-2ae7-4af9-b9fa-ecf79907b185@f21g2000prn.googlegroups.com>,
RichD <r_delaney2001@yahoo.com> wrote:

This may be a dumb question, but anyway....

A lens focuses energy, does it not? That's its
purpose. So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

Does this not violate the 2nd law of thermodynamics?
The natural evolution is the radiation spreads,
becoming more disorderly and lower temperature.
The lens reverse this process, concentrating the
energy, with temperature rise.

And the lens is a passive device, so no exogenous
power source to do the work.

Ot it could be a parabolic mirror, same thing.

Explanation?

--
Rich

If you look at an object through a lens, it is likely that you will see
an image of the object. If you do, it will not (except in rare
circumstances) be the same size as the object or in the same location as
the object. Nevertheless, the image will be the same brightness as the
object except for some losses such as reflection at glass surfaces.

In particular, note that as you stop down a lens, illumination by the
image formed in the focal plane diminishes. That is one means of
exposure control. However, if you observe the image by eye, it is as
bright as the source irrespective of reducing the effective diameter of
the lens with a variable iris. The brightness remains the same, but your
field of view is restricted so that you may not be able to see all of
the object. When you go through all of the geometry, the lens cannot
collect all of the light emitted by a source or produce a temperature
higher than that of a source.

Bill

--
An old man would be better off never having been born.

 

[Jasen Betts]

On 2011-01-24, RichD <r_delaney2001@yahoo.com> wrote:

This may be a dumb question, but anyway....

A lens focuses energy, does it not?

sometimes it does.

That's its
purpose. So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

if aligned correctly it does.

Does this not violate the 2nd law of thermodynamics?

no.

The natural evolution is the radiation spreads,

yes (not sure what you mean)

becoming more disorderly and lower temperature.

no

The lens reverse this process, concentrating the
energy, with temperature rise.

the nearest thing light has to temperature is frequency and
that's not reduced by dispersal.

And the lens is a passive device, so no exogenous
power source to do the work.

Ot it could be a parabolic mirror, same thing.

--
⚂⚃ 100% natural

 

[Androcles]

"RichD" <r_delaney2001@yahoo.com> wrote in message
news:3a96de31-2ae7-4af9-b9fa-ecf79907b185@f21g2000prn.googlegroups.com...
| This may be a dumb question, but anyway....
|
| A lens focuses energy, does it not? That's its
| purpose. So given a large radiating source, the
| lens concentrates a dissipating spreading wave,
| down to a small area.
|
| Does this not violate the 2nd law of thermodynamics?
| The natural evolution is the radiation spreads,
| becoming more disorderly and lower temperature.
| The lens reverse this process, concentrating the
| energy, with temperature rise.
|
| And the lens is a passive device, so no exogenous
| power source to do the work.
|
| Ot it could be a parabolic mirror, same thing.
|
| Explanation?
|
| --
| Rich
|
The energy comes from the source (usually the Sun), the lens concentrates it
to light a boy scout's fire, the fire gets out of control and burns down the
forest. This dissipates the energy that the forest collected from the Sun
through its leaves and stored as potential energy, wood you can burn or
compress underground as coal and burn later. Continued concentration of
sunlight on the remaining ash from the fire only heats the ash. Your lens
has accelerated entropy rather than reversed it. Living matter which
concentrates energy in an organised way is the real reverser of entropy, but
chaos wins in the end. Even your lens used entropy in its manufacture,
melting the glass and grinding the shape.
It takes energy to concentrate energy.

 

[RL]

Look up conservation of brightness

"RichD" <r_delaney2001@yahoo.com> wrote in message
news:3a96de31-2ae7-4af9-b9fa-ecf79907b185@f21g2000prn.googlegroups.com...

This may be a dumb question, but anyway....

A lens focuses energy, does it not? That's its
purpose. So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

Does this not violate the 2nd law of thermodynamics?
The natural evolution is the radiation spreads,
becoming more disorderly and lower temperature.
The lens reverse this process, concentrating the
energy, with temperature rise.

And the lens is a passive device, so no exogenous
power source to do the work.

Ot it could be a parabolic mirror, same thing.

Explanation?

--
Rich

 

[Sam Wormley]

Brightness and Flux Density
http://www.cv.nrao.edu/course/astr534/Brightness.html

 

[John Larkin]

On Sun, 23 Jan 2011 18:39:40 -0600, Sam Wormley <swormley1@gmail.com>
wrote:

On 1/23/11 6:11 PM, RichD wrote:
This may be a dumb question, but anyway....

A lens focuses energy, does it not? That's its
purpose. So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

Does this not violate the 2nd law of thermodynamics?
The natural evolution is the radiation spreads,
becoming more disorderly and lower temperature.
The lens reverse this process, concentrating the
energy, with temperature rise.

Focus the sun's energy coming through the earth's
atmosphere. The achievable temperature at the focal
point is is always less than that of the source, in
this case, the surface of the sun.

There was an article in SciAm, ages ago, about a non-imaging optical
system that could heat an object to above the sun's surface
temperature. Sounds fishy, except that the sun does emit a lot of UV,
energy past the black-body peak wavelength, which could be useful if
concentrated enough. Like, 84,000:1.

http://en.wikipedia.org/wiki/Non-imaging_optics

I guess one could always use a sun-powered laser to heat an object to
an arbitrarily absurd temperature. One of the early "theoretical"
objections to funding Townes' maser/laser idea is that it obvioulsy
violated the 2nd law.

John

 

[whit3rd]

On Jan 23, 4:11 pm, RichD <r_delaney2...@yahoo.com> wrote:

A lens focuses energy, does it not?  That's its
purpose.   So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

Does this not violate the 2nd law of thermodynamics?

No, not unless the focus could be heated to a higher
temperature than the light source. When you use a magnifier
to focus sunlight on a weed, the view (from the weed)
is of not just 1/2 degree of the sky being hot and yellow,
the rest blue and cool, but of a 20 degree segment of
the sky (the full width of the lens) being hot and yellow.
If you got the weed close to the sun and put a mirror behind
it, it'd get the full effect, and reach the temperature of
the sun's photosphere. But never a higher temperature.

Mirrors to focus sunlight from all directions onto a point would
have the effect of removing radiative thermal connection to all
local objects, while retaining the connection to the sun. That
would make the poor weed reach equilibrium when it hit 5800K or so.

 

[Salmon Egg]

In article <v2srj6lq8ds46k0mg7jb0u484b18ll1atu@4ax.com>,
John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

I guess one could always use a sun-powered laser to heat an object to
an arbitrarily absurd temperature. One of the early "theoretical"
objections to funding Townes' maser/laser idea is that it obvioulsy
violated the 2nd law.

This is just an instance of a common situation where local entropy is
reduced at the expense of increased overall entropy. It happens in
humans by eating food as well as in refrigerators or heat pumps.

Part of the overall problem in this discussion is poor use of language.
What does it mean to focus? Why would one expect focussing to violate
the second law? Vernacular use of language easily gets you into trouble.
Explain to me what is grammatical about the phrase "twice as small."

Bill

--
An old man would be better off never having been born.

 

[Androcles]

--
*plonk*

Do not reply to this generic message, it was automatically generated;
you have been kill-filed, either for being boringly stupid, repetitive,
unfunny, ineducable, repeatedly posting politics, religion or off-topic
subjects to a sci. newsgroup, attempting cheapskate free advertising
for profit, because you are a troll, because you responded to George
Hammond the complete fruit cake, simply insane or any combination
or permutation of the aforementioned reasons; any reply will go unread.

Boringly stupid is the most common cause of kill-filing, but because
this message is generic the other reasons have been included. You are
left to decide which is most applicable to you.

There is no appeal, I have despotic power over whom I will electronically
admit into my home and you do not qualify as a reasonable person I would
wish to converse with or even poke fun at. Some weirdoes are not kill-
filed, they amuse me and I retain them for their entertainment value
as I would any chicken with two heads, either one of which enables the
dumb bird to scratch dirt, step back, look down, step forward to the
same spot and repeat the process eternally.

This should not trouble you, many of those plonked find it a blessing
that they are not required to think and can persist in their bigotry
or crackpot theories without challenge.

You have the right to free speech, I have the right not to listen. The
kill-file will be cleared annually with spring cleaning or whenever I
purchase a new computer or hard drive.
Update: the last clearance was 19/08/10. Some individuals have been
restored to the list.

I'm fully aware that you may be so stupid as to reply, but the purpose
of this message is to encourage others to kill-file fuckwits like you.

I hope you find this explanation is satisfactory but even if you don't,
damnly my frank, I don't give a dear. Have a nice day and fuck off.



"RL" <rlombardo@cox.net> wrote in message
news:ihkjol$cea$1@news.eternal-september.org...
| Look up conservation of brightness
|
| "RichD" <r_delaney2001@yahoo.com> wrote in message
| news:3a96de31-2ae7-4af9-b9fa-ecf79907b185@f21g2000prn.googlegroups.com...
| > This may be a dumb question, but anyway....
| >
| > A lens focuses energy, does it not? That's its
| > purpose. So given a large radiating source, the
| > lens concentrates a dissipating spreading wave,
| > down to a small area.
| >
| > Does this not violate the 2nd law of thermodynamics?
| > The natural evolution is the radiation spreads,
| > becoming more disorderly and lower temperature.
| > The lens reverse this process, concentrating the
| > energy, with temperature rise.
| >
| > And the lens is a passive device, so no exogenous
| > power source to do the work.
| >
| > Ot it could be a parabolic mirror, same thing.
| >
| > Explanation?
| >
| > --
| > Rich
|
|
|

 

[boxman]

There was an article in SciAm, ages ago, about a non-imaging optical
system that could heat an object to above the sun's surface
temperature. Sounds fishy, except that the sun does emit a lot of UV,
energy past the black-body peak wavelength, which could be useful if
concentrated enough. Like, 84,000:1.

Actually to be fair, the ratio you list (84,000:1) is the concentration
ratio provided by the system which can produce irradiance levels higher
than the sun. There are no tricks used in regards to using UV energy or
something like that. The absorber is simply immersed in an index of
refraction that is different from air. The temperature of the abosrber
reached however will never exceed that of the sun as a blackbody since
the absorber immersed in a higher refractive index also radiates more
than one immersed in air, thus the temperatures remain equal in theory.
I don't think the SciAm article lists the temperature as being
higher, I think it talks about the irradiance being higher, which are
two different quantities. See the following link for further details

http://www.solideas.com/papers/GPS_ThermCon_SEMSC.pdf

 

[Sequoia Sempervirens]

"RichD" <r_delaney2001@yahoo.com> wrote in message
news:3a96de31-2ae7-4af9-b9fa-ecf79907b185@f21g2000prn.googlegroups.com...

This may be a dumb question, but anyway....

A lens focuses energy, does it not? That's its
purpose. So given a large radiating source, the
lens concentrates a dissipating spreading wave,
down to a small area.

Does this not violate the 2nd law of thermodynamics?
The natural evolution is the radiation spreads,
becoming more disorderly and lower temperature.
The lens reverse this process, concentrating the
energy, with temperature rise.

And the lens is a passive device, so no exogenous
power source to do the work.

Ot it could be a parabolic mirror, same thing.

Explanation?

--
Rich

I am not sure I understand what you mean (and I will not
try to), you might however study, if you can, the relevant
chapter from Clausius famous treatise.

It all boils down to the conservation of "étendue" in
optical systems...

Rudolf Clausius, The Mechanical Theory of Heat etc.,
Eighth Memoir, On the Concentration of Rays of Heat
and Light etc., London 1868.

**Can be downloaded free from Google Books**

There might be simpler accounts elsewhere, I do not know
of any in English, there could be a couple in French I think.

Some people say that Clausius derivations (might) contain
errors however the results are not in question.