thought for the day which is better

rogerblack

Shuttle tiles are basically irrelevant to this - as they are optimised for rejecting heat at ~2000C, or so, rather hotter than most want their radiators.
Back to the question.

In respect of radiating heat through infra-red, the 'colour' you care about is not the red/green/blue that the eye sees, but the much longer infrared wavelengths.
All things glow with radiated heat - but the amount of glow depends on their emissivity.
Emissivity is another way of saying absorbtion, but in the other direction.

Heated to 1000C, glass will be a dim glow in a dark room, but graphite will be comparatively brilliant - the graphite has a really high emissivity, and the glass a low.

When it comes to lower temps - not 'red hot' or 'white hot', but around 40C, the light emitted - infrared - is so far from visible that the 'binder' of the paint - the clear film that surrounds the paint particles - is not transparent.
So, instead of white and black paints being very different, they are all much the same.

Bare metal is lots worse - it emits comparatively little light, especially if clean.
http://en.wikipedia.org/wiki/Emissivity

victor2

energysavingexp wrote: »

yes but if you shine a light into space you cannot see the beam you can only see the reflection off an object/partical
in space you dont see the beam of light from the stars do you. you only see the object :rotfl:

You would see it if it was coming your way - as it does if it's reflected off something that bounces a beam at you....:)

energysavingexp

victor2 wrote: »

You would see it if it was coming your way - as it does if it's reflected off something that bounces a beam at you....:)

but if you shine a light so you are looking across the beam you dont see the light if its comming your way lets say from a star that is one light year away when you look at it you dont actuall see the star all you are looking at is the reflection of light wave that left that star one year ago so in effect you are looking back in time one year and not seeing what is actuall there

victor2

energysavingexp wrote: »

but if you shine a light so you are looking across the beam you dont see the light if its comming your way lets say from a star that is one light year away when you look at it you dont actuall see the star all you are looking at is the reflection of light wave that left that star one year ago so in effect you are looking back in time one year and not seeing what is actuall there

Exactly. And as Einstein said, nothing can travel faster than light, so we'll never see the star in "real" time unless we go there. But then those swiss chappies and their little collider thingy seem to be set to disprove that.

energysavingexp

rogerblack wrote: »

Shuttle tiles are basically irrelevant to this - as they are optimised for rejecting heat at ~2000C, or so, rather hotter than most want their radiators.
Back to the question.

In respect of radiating heat through infra-red, the 'colour' you care about is not the red/green/blue that the eye sees, but the much longer infrared wavelengths.
All things glow with radiated heat - but the amount of glow depends on their emissivity.
Emissivity is another way of saying absorbtion, but in the other direction.

Heated to 1000C, glass will be a dim glow in a dark room, but graphite will be comparatively brilliant - the graphite has a really high emissivity, and the glass a low.

When it comes to lower temps - not 'red hot' or 'white hot', but around 40C, the light emitted - infrared - is so far from visible that the 'binder' of the paint - the clear film that surrounds the paint particles - is not transparent.
So, instead of white and black paints being very different, they are all much the same.

Bare metal is lots worse - it emits comparatively little light, especially if clean.
http://en.wikipedia.org/wiki/Emissivity

thanks for that but if the colour of paint is the same why is it that a white/silver object is a lot cooler when light is shinned on it and the black object is a lot hotter surly the black one is absorbing the light and turning to heat is it not love to know the right answer

energysavingexp

victor2 wrote: »

Exactly. And as Einstein said, nothing can travel faster than light, so we'll never see the star in "real" time unless we go there. But then those swiss chappies and their little collider thingy seem to be set to disprove that.

yeh i watched it it was with prof Brian Cox very intresting even though most of it went streight over my head

its like the old saying if you travelled at the speed of light time stands still wow:j

grahamc2003

energysavingexp wrote: »

thought for the day which is better:

which is more Efficient a bare metal radiator. one sprayed sliver or one sprayed black
the reason for the question is will the silver or black paint act as an insulator there for not emitting as much heat.
or will the silver one emit more as silver reflects heat
or the black one which absorbs heat?

ps: is it true you cannot see light but you can only see an illuminated reflection of an object as light travels through space its only the refletion off dust particals or objects that you see and not the light its self?

Not sure what you mean by effcient when referring to a radiator, but if you are asking how a radiator gives out the most radiation, then that is by being painted black.

Yes, silver reflects heat, so it will reflect the radiant heat back into the radiator, so overall, the radiator will radiate less heat when covered in silver foil.

The black will both radiate more and absorb more radiant energy than other colours. It's just that usually a radiator is hotter than the rest of the environment, so the extra outward radiation will be greater than the extra inward radiation. (You have to realise that everything both radiates heat and absorbs radiated heat from every other body. The radiant energy is a function of the absolute temperature to the power 4, so a small increase in temperature results in a high increase in radiated energy).

Regarding light. You see things by photons entering your eye. The photons get there by bouncing off things. The room you are in is full of photons going all over the place (in other words, you can put your eye anywhere and you'll form an image), so in that sense you can't see them, and the image you see is the effect of those photons on your retina and the signals your retina generates and how your brain deals with the signals.

As an aside, if you hang upside down for a few days, what you initially see will appear upside down, but after a while, your brain will make the images appear the right way up. So what we think we see is very much our brain's interpretaion of its environment.

energysavingexp

grahamc2003 wrote: »

Not sure what you mean by effcient when referring to a radiator, but if you are asking how a radiator gives out the most radiation, then that is by being painted black.

Yes, silver reflects heat, so it will reflect the radiant heat back into the radiator, so overall, the radiator will radiate less heat when covered in silver foil.

The black will both radiate more and absorb more radiant energy than other colours. It's just that usually a radiator is hotter than the rest of the environment, so the extra outward radiation will be greater than the extra inward radiation. (You have to realise that everything both radiates heat and absorbs radiated heat from every other body. The radiant energy is a function of the absolute temperature to the power 4, so a small increase in temperature results in a high increase in radiated energy).

Regarding light. You see things by photons entering your eye. The photons get there by bouncing off things. The room you are in is full of photons going all over the place (in other words, you can put your eye anywhere and you'll form an image), so in that sense you can't see them, and the image you see is the effect of those photons on your retina and the signals your retina generates and how your brain deals with the signals.

As an aside, if you hang upside down for a few days, what you initially see will appear upside down, but after a while, your brain will make the images appear the right way up. So what we think we see is very much our brain's interpretaion of its environment.

thanks for that grahamc
but as you are well better placed than me in this knolage than me (i skipped a few lessons )
is it not true that a radiator does not radiate heat it convects heat
as i was told may it be wrong that conduction is direct contact ie: a pan on a element with no air space is conduction and convection is heat that requires air or water to pass on the heat to an object,
and radiation is electromagneticwaves requiring no direct contact ie: me sun bathing in the garden is radiation so why is a radiator called a radiator when it dosnt radiate heat it convects heat should it not have been called a convector not a rad ?

grahamc2003

energysavingexp wrote: »

thanks for that grahamc
but as you are well better placed than me in this knolage than me (i skipped a few lessons )
is it not true that a radiator does not radiate heat it convects heat
as i was told may it be wrong that conduction is direct contact ie: a pan on a element with no air space is conduction and convection is heat that requires air or water to pass on the heat to an object,
and radiation is electromagneticwaves requiring no direct contact ie: me sun bathing in the garden is radiation so why is a radiator called a radiator when it dosnt radiate heat it convects heat should it not have been called a convector not a rad ?

Well radiators (the one's connected to CH systems) heat the room both by convection (i.e. heating a body of air which then rises and cirulates) and radiation - istr the ratio is typically 70% convection and 30% radiation (so, yes, they are misnamed, as you imply). One's painted black will have higher radiation percentage than those painted white (so it's strange that mostly, radiators are white!).

Just a point on radiation in general, everything - you, me, dust, elephants, the sun, the moon, light bulbs, solid carbon dioxide - both emit (to everything else) and absorb (from everything else) radiation.