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Effectiveness of a white shield against sunlight

Effectiveness of a white shield against sunlight - Physics Forum

Effectiveness of a white shield against sunlight - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 01-02-2007, 03:35 AM
nimmi_srivastav@yahoo.com
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Default Effectiveness of a white shield against sunlight



If I were to pitch a huge airtight white colored pavilion on a bright
sunny day and seek refuge under it, would the heat from the sun be
completely blocked for me? From what I have read in high school, a
white colored body reflects all the colors of the spectrum, and does
not absorb any color. In accordance with that theory, all energy from
the sun would be reflected back (unless I misinterpreted the theory).
However, I think that due to conduction, heat would be transferred from
the outer surface of the pavilion to the inner surface of the
pavilion. This would, in turn, set up convection currents inside the
pavilion, ensuring some heat circulation.

Am I correct, or completely off the mark?

Thanks,
Nimmi

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  #2  
Old 01-02-2007, 03:46 AM
Sam Wormley
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Default Effectiveness of a white shield against sunlight

[Only registered users see links. ] wrote:

No, but for most materials a lot of the infrared wavelengths would
be blocked.


From what I have read in high school, a
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  #3  
Old 01-02-2007, 03:46 AM
Sam Wormley
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Default Effectiveness of a white shield against sunlight

[Only registered users see links. ] wrote:

No, but for most materials a lot of the infrared wavelengths would
be blocked.


From what I have read in high school, a
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  #4  
Old 01-02-2007, 03:54 AM
Sam Wormley
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Default Effectiveness of a white shield against sunlight

[Only registered users see links. ] wrote:

Note that there exist glass (often metalized glass) that block
a lot of infrared wavelengths while passing most of the visible
spectrum.

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  #5  
Old 01-02-2007, 03:54 AM
Sam Wormley
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Default Effectiveness of a white shield against sunlight

[Only registered users see links. ] wrote:

Note that there exist glass (often metalized glass) that block
a lot of infrared wavelengths while passing most of the visible
spectrum.

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  #6  
Old 01-02-2007, 07:28 AM
N:dlzc D:aol T:com \(dlzc\)
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Default Effectiveness of a white shield against sunlight

Dear Sam Wormley:

"Sam Wormley" <[Only registered users see links. ]> wrote in message
news:vHkmh.199633$aJ.193110@attbi_s21...
....

Actually, standard "soda lime glass" (which appears green on
edge) is largely transparent to visible light, and opaque to both
UV and infrared. Part of the reason that the inside of a car
gets so hot... The transmitted visible light has only a 50-50
chance of getting back out through glass... once it becomes heat.

David A. Smith


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  #7  
Old 01-02-2007, 07:28 AM
N:dlzc D:aol T:com \(dlzc\)
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Default Effectiveness of a white shield against sunlight

Dear Sam Wormley:

"Sam Wormley" <[Only registered users see links. ]> wrote in message
news:vHkmh.199633$aJ.193110@attbi_s21...
....

Actually, standard "soda lime glass" (which appears green on
edge) is largely transparent to visible light, and opaque to both
UV and infrared. Part of the reason that the inside of a car
gets so hot... The transmitted visible light has only a 50-50
chance of getting back out through glass... once it becomes heat.

David A. Smith


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  #8  
Old 01-02-2007, 07:51 AM
Timo A. Nieminen
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Default Effectiveness of a white shield against sunlight

On Tue, 2 Jan 2007, N:dlzc D:aol T:com (dlzc) wrote:


That's IR, sure. But near IR goes through many glasses quite well. What
are optical fibres made of (eg for wavelengths of 1.3 microns)?
Just-into-IR goes through microscope objectives very well, though we lose
about 50% at 1064nm/1070nm.

--
Timo Nieminen - Home page: [Only registered users see links. ]
E-prints: [Only registered users see links. ]
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  #9  
Old 01-02-2007, 07:51 AM
Timo A. Nieminen
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Default Effectiveness of a white shield against sunlight

On Tue, 2 Jan 2007, N:dlzc D:aol T:com (dlzc) wrote:


That's IR, sure. But near IR goes through many glasses quite well. What
are optical fibres made of (eg for wavelengths of 1.3 microns)?
Just-into-IR goes through microscope objectives very well, though we lose
about 50% at 1064nm/1070nm.

--
Timo Nieminen - Home page: [Only registered users see links. ]
E-prints: [Only registered users see links. ]
Shrine to Spirits: [Only registered users see links. ]

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  #10  
Old 01-02-2007, 07:58 AM
Timo A. Nieminen
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Default Effectiveness of a white shield against sunlight

On Tue, 1 Jan 2007, [Only registered users see links. ] wrote:


A white body reflects _most_ of all of the _visible_ colours of the
spectrum. We receive non-visible parts of the spectrum from the sun, too.
Also, "white" doesn't mean perfectly reflective, just enough to look white
rather than grey. Still, in an ideal world, where it would be white at all
wavelengths, and perfectly reflecting, yes, you're right.

Except for heat/light that will be radiated/reflected by all the things to
the sides. Still, it can be a big improvement.

About the closest cheap material to a perfect reflector at the wavelengths
of interest is polished aluminium. However, it isn't necessarily a good
thing to cover your pavillion with. It isn't a perfect reflector, so the
sunlight will heat it up. If it can cool down by radiating infra-red
radiation, then it won't get too hot, and the air under it won't get too
hot. However, it's even more reflective in thermal IR than in the visible,
so it doesn't cool down easily. White paint (and probably most white
cloth) is reflective in the visible, and a good emitter (ie poor
reflector) in thermal IR, so the underside of a white painted roof is
cooler than an aluminium roof.

Even better would be painted white on top, bare aluminium on the
underside.

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