Black Body

Black Body
https://en.wikipedia.org/wiki/Black_body
"An ideal body is now defined, called a blackbody. A blackbody allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body). This is true for radiation of all wavelengths and for all angles of incidence. Hence the blackbody is a perfect absorber for all incident radiation.[10]"

A star or planet often is modeled as a black body, and electromagnetic radiation emitted from these bodies as black-body radiation.

My question is as follow:
Can we assume that any star, planet, moon would be considered as a black body, even if it is a simple gas star? In other words, is there any limit in its size or its radiation/energy density in order to be considered as black body?

Dne 03/12/2016 v 09:46 David Levy napsal(a):
[....]

A star or planet often is modeled as a black body, and electromagnetic
radiation emitted from these bodies as black-body radiation.

My question is as follow:
Can we assume that any star, planet, moon would be considered as a black
body, even if it is a simple gas star? In other words, is there any
limit in its size or its radiation/energy density in order to be
considered as black body?

An object can be considered as a black body (BB),
if it has for the wavelength a/o frequency range of interest
the negligible reflectance.

E.g. the Earth can be considered as approximately BB
for the thermal IN range, but not for the visible range.

The closest approximation to the BB is a hole into a cavity.
Reflection or transmission of radiation coming into the cavity
through the smal enough hole is not measurable.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.

Originally Posted by Poutnik[_5_]

The closest approximation to the BB is a hole into a cavity.
Reflection or transmission of radiation coming into the cavity
through the smal enough hole is not measurable.

Please look at the following diagram:
https://en.wikipedia.org/wiki/Black_...ealization.png
It is stated:
"An approximate realization of a black body as a tiny hole in an insulated enclosure"

Why do we need this tiny hole/cavity?
Is it needed just to place the first radiation at that insulated enclosure?
Let's assume that the insulated enclosure is isolated by 100%.
Let's also assume that we have the ability to close that tiny hole at the moment that the first radiation gets in.
So, theoretically, the energy of the radiation will bump the walls forever and at the same amplitude.
Now, if after some time we will measure that radiation.
Would it have a black body signature?

Dne 04/12/2016 v 13:30 David Levy napsal(a):
'Poutnik[_5_ Wrote:
;1328995']
The closest approximation to the BB is a hole into a cavity.
Reflection or transmission of radiation coming into the cavity
through the smal enough hole is not measurable.


Please look at the following diagram:
http://tinyurl.com/lfnwltj
It is stated:
"An approximate realization of a black body as a tiny hole in an
insulated enclosure"

Why do we need this tiny hole/cavity?
Is it needed just to place the first radiation at that insulated
enclosure?
Let's assume that the insulated enclosure is isolated by 100%.
Let's also assume that we have the ability to close that tiny hole at
the moment that the first radiation gets in.
So, theoretically, the energy of the radiation will bump the walls
forever and at the same amplitude.
Now, if after some time we will measure that radiation.
Would it have a black body signature?

The idea is easy, incoming radiation gets absorbed
by repeated non ideal absorption. That object behaving like this
actsas BB, followingn the thermodynamic laws.

As if not, there would not be possible the radiative equilibrium.

The is no sense ion closing it.
Neither the incoming rafiation would bounce forever,
but would be absorbed quickly.

The BB thermal radiation within the cavity is another topic.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.

Originally Posted by Poutnik[_5_]

The idea is easy, incoming radiation gets absorbed
by repeated non ideal absorption. That object behaving like this
actsas BB, followingn the thermodynamic laws.

As if not, there would not be possible the radiative equilibrium.

The is no sense ion closing it.
Neither the incoming rafiation would bounce forever,
but would be absorbed quickly.

The BB thermal radiation within the cavity is another topic.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.

Thanks

That is clear.

Originally Posted by 'Poutnik[_5_

The is no sense ion closing it.
Neither the incoming rafiation would bounce forever, but would be absorbed quickly.

However, if it had been absorbed quickly, then by the time that we will try to measure it, we might find that its amplitude is already zero.
So, let's assume that we have the ability to generate a constant radiation in that enclosure.
Not just one source of radiation, but as much as needed.
Let's also assume that the size of those sources is less than photon. (So technically, they won't interfere with the bouncing process).
In this case, do you estimate that we could get a constant BB signature from that aggregated radiation?

Dne 05/12/2016 v 15:40 David Levy napsal(a):
'Poutnik[_5_ Wrote:

The is no sense ion closing it.
Neither the incoming rafiation would bounce forever, but would be
absorbed quickly.



However, if it had been absorbed quickly, then by the time that we will
try to measure it, we might find that its amplitude is already zero.
So, let's assume that we have the ability to generate a constant
radiation in that enclosure.
Not just one source of radiation, but as much as needed.
Let's also assume that the size of those sources is less than photon.
(So technically, they won't interfere with the bouncing process).
In this case, do you estimate that we could get a constant BB signature
from that aggregated radiation?


I am not sure what you try to achieve.

BB radiation is radiation belonging to radiative equilibrium
maintained at given temperature.

At given temperature, all radiation is absorbed by BB,
and at the same time the same radiaton is emitted by BB,
to maintain zero net energy flow of the equilibrium.

If black body and generara body are at radiative equilibrium,
the portion that is not absorbed, is not emitted,
so incoming and outgoing flows are the same as for BB.




--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.

Dne 03/12/2016 v 09:46 David Levy napsal(a):


My question is as follow:
Can we assume that any star, planet, moon would be considered as a black
body, even if it is a simple gas star?

What do you mean by a simple gas star ?
It is either gas, either star.
Plasma is not gas, and has high rate of absoption.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.

David Levy wrote:
Black Body
https://en.wikipedia.org/wiki/Black_body
"An ideal body is now defined, called a blackbody. A blackbody allows
all incident radiation to pass into it (no reflected energy) and
internally absorbs all the incident radiation (no energy transmitted
through the body). This is true for radiation of all wavelengths and
for all angles of incidence. Hence the blackbody is a perfect
absorber for all incident radiation.[10]"

A star or planet often is modeled as a black body, and electromagnetic
radiation emitted from these bodies as black-body radiation.

My question is as follow:
Can we assume that any star, planet, moon would be considered as a
black body, even if it is a simple gas star? In other words, is there
any limit in its size or its radiation/energy density in order to be
considered as black body?

A star should not be considered a black body because the light emerging is
passing through an atmosphere with lots of absorption lines and variations
of opacity with wavelength. The nearest thing to a BB for a star would be
one that has a "grey atmosphere" with constant fractional absorption
coefficient independent of wavelength. That would not be quite like a BB as
the spectrum would be less peaked at maximum wavelength. The closest thing
to this is probably a very hot star where electron scattering dominates the
opacity.

A planet or moon or asteroid is a reasonable approximation to a BB if it
does not have an atmosphere, and if it does, the overall emission of energy
will balance the absorption of energy if you take into account all the
sources of opacity and reflection (albedo) at different wavelengths.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

Dne 03/12/2016 v 17:48 Mike Dworetsky napsal(a):


A planet or moon or asteroid is a reasonable approximation to a BB if it
does not have an atmosphere, and if it does, the overall emission of
energy will balance the absorption of energy if you take into account
all the sources of opacity and reflection (albedo) at different
wavelengths.


Hm, this is refuted by huan eyes look.
Earth in visible region is far from black body behavior.

The very most surfaces are far from be anywhere near the black.

In the thermal IR region near 10 um
Earth is much closer to BB, especially oceans.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.