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  #1  
Old 01-08-2012, 08:47 AM
constanze constanze is offline
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Does a mirror reflect all light including IR?

Okay, the following hypothetical scenario*

Bob is sitting on the beach in sunshine in warm weather, shirt off. The sun shines on him, but his back is in his shadow (he's casting himself).

Alice sits behind him with a) a mirror b) a watch and uses this to reflect a circle of sunlight onto his back.

Will Bob be able to feel this?

1) Yes, a mirror will reflect all light, visible + IR

2) Yes, both mirror and watch will reflect IR

3) Yo** - the mirror will reflect IR, but due to background IR = the high temp. on a sunshine day on the beach, Bob won't feel the small heat from the reflection.

4) Yo** - it depends on the quality of the mirror whether it will reflect IR in addition to visible light

5) No - IR is too scattered/ short to begin with to be reflected by a normal mirror

6) .... another option

* Which I can't test myself currently due to lack of sunshine...

** Yes and no

Experimental answers would be most welcome, because I can imagine both Yes and no from theory.
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  #2  
Old 01-08-2012, 09:01 AM
Arkcon Arkcon is offline
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If you go to the Edmund Scientific website, you'll see that they sell "hot mirrors" and "cold mirrors" -- mirrors specifically designed to reflect or pass more of the wavelengths associated with IR. And they give that info to you down to the nanometer. I guess, the average silver backed glass hand mirror doesn't reflect as much IR, because the very best hot mirrors are front-faced with gold, with a molecule thick silica coating for protection. You kinda figure that -- gold being visually "redder" than silver would reflect more.
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  #3  
Old 01-08-2012, 09:16 AM
njtt njtt is offline
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Well, you can set things on fire with a concave mirror, presumably because it is focusing IR. My mother once had a concave make-up mirror on a stand on her dressing table. It caught the rays of sunlight coming in through the window and burned holes in the curtains.
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  #4  
Old 01-08-2012, 09:18 AM
Candyman74 Candyman74 is offline
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People use those funny mirror things around their necks to tan under their chins, so certainly some UV light is reflected. I've no idea how efficient it is, and whether that includes IR, too.

I'd guess yes - if it reflects visible light and UV light, it seems odd that it wouldn't reflect IR, too. I wonder how far up and down the spectrum it goes? Radio waves? Microwaves?
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  #5  
Old 01-08-2012, 09:29 AM
beowulff beowulff is online now
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It all depends on what the mirror is made from.
Most metals have excellent reflectivity in the IR, so that's not an issue. SHorter wavelengths are more of a problem.
Here's a chart of some coating:
http://www.molalla.com/members/leeper/refcoat.pdf
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  #6  
Old 01-08-2012, 09:48 AM
constanze constanze is offline
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Quote:
Originally Posted by njtt View Post
Well, you can set things on fire with a concave mirror, presumably because it is focusing IR. My mother once had a concave make-up mirror on a stand on her dressing table. It caught the rays of sunlight coming in through the window and burned holes in the curtains.
Isn't that done with visible light only, too?

How much IR can get through window glass in the first place?
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  #7  
Old 01-08-2012, 09:50 AM
constanze constanze is offline
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Originally Posted by Candyman74 View Post
People use those funny mirror things around their necks to tan under their chins, so certainly some UV light is reflected. I've no idea how efficient it is, and whether that includes IR, too.
But UV is on the other end of the spectrum (blue, not red) and very powerful waves, whereas Infrared is already weak and scattered light. It's closer to heat, which is scattered and therefore directionless

Quote:
I'd guess yes - if it reflects visible light and UV light, it seems odd that it wouldn't reflect IR, too. I wonder how far up and down the spectrum it goes? Radio waves? Microwaves?
Radio and Microwaves obviously depend on the type of material of mirror - see the dishes for radar, satellite etc.
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  #8  
Old 01-08-2012, 09:53 AM
chrisk chrisk is offline
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Quote:
Originally Posted by constanze View Post
Isn't that done with visible light only, too?

How much IR can get through window glass in the first place?
Visible light could burn, but because of the wider wavelength available, IR can convey more heat and do it more easily.

And I believe that a lot of IR gets through ordinary glass - it's at the other end, with UV light, that it starts to become opaque.
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  #9  
Old 01-08-2012, 10:01 AM
notsoheavyd3 notsoheavyd3 is offline
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Originally Posted by constanze View Post
Isn't that done with visible light only, too?
If it didn't work with visible light that'd break conservation of energy and the second law of thermodynamics. (I think people think that because too many have heard that thermal radiation = infared.)
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  #10  
Old 01-08-2012, 10:48 AM
engineer_comp_geek engineer_comp_geek is online now
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Quote:
Originally Posted by constanze View Post
Isn't that done with visible light only, too?
Yes.

Quote:
Originally Posted by constanze View Post
How much IR can get through window glass in the first place?
It varies with the frequency of the IR, the exact type of glass, etc. For your typical Joe Average window pane and the frequencies that an IR remote works at, the glass stops about 50 percent of the IR from getting through.
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  #11  
Old 01-08-2012, 10:51 AM
hajario hajario is offline
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Quote:
Originally Posted by engineer_comp_geek View Post
It varies with the frequency of the IR, the exact type of glass, etc. For your typical Joe Average window pane and the frequencies that an IR remote works at, the glass stops about 50 percent of the IR from getting through.
IR cameras can't see through window glass. They see reflected images in window glass.
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  #12  
Old 01-08-2012, 11:09 AM
notsoheavyd3 notsoheavyd3 is offline
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Originally Posted by hajario View Post
IR cameras can't see through window glass. They see reflected images in window glass.
Funny, I just tested this a second ago with the remote for my TV and using a standard digital camera. (Since pretty much any digital camera can see near infra red.) Basically pointed my camera on one side of a piece of glass at the remote on the other and pushed the volume up button. Yes, I could see the light light up on the camera.
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  #13  
Old 01-08-2012, 11:28 AM
beowulff beowulff is online now
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Window glass blocks about 60% of near IR, so that's only about 1 stop underexposure for a camera.
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  #14  
Old 01-08-2012, 11:43 AM
Chronos Chronos is online now
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When you feel the heat from the Sun, that's mostly the visible light you're feeling anyway, not the infrared. Yes, infrared light can carry heat, but so can any light at all, via the exact same mechanism. The only reason people specifically associate infrared with heat is that most of the objects we're familiar with (animal bodies, fires, stove burners, old-fashioned light bulbs, etc.) happen to be at a cool enough temperature that most of their radiation is in the infrared range. But the Sun isn't "most objects", and is much hotter, hot enough that most of its radiation is in the visible range.

And since the OP said all light, it's easy to make a mirror that will work reasonably well in visible, IR, and near UV (in fact, most mirrors will work reasonably well in that range without even being specifically designed for it), but beyond that, not necessarily. So far as I know, there's no known way to make a gamma-ray mirror at all, for instance, and for far UV or X-rays, you usually have to settle for only a few percent reflectivity.
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  #15  
Old 01-08-2012, 12:13 PM
JWT Kottekoe JWT Kottekoe is offline
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Quote:
Originally Posted by constanze View Post
... Infrared is already weak and scattered light. It's closer to heat, which is scattered and therefore directionless ...
When I was a student, I harbored a misconception that infrared radiation was a form of heat. I think I learned this in elementary school and it became a great source of confusion to me for a few years. IR is just like any other wavelength of light. It has no more or less to do with heat than visible light.

The energy we receive from the Sun is essentially thermal radiation. The Sun is a hot ball whose surface temperature determines the spectral distribution of this thermal radiation. Because the Sun is so hot, it's thermal radiation peaks in the visible. At sea level, about 52% of the Sun's radiation is within the visible range. About 42% is within the IR range, and the remaining 6% above and below that. The thermal radiation from room temperature objects peaks in the infrared and I think this is where the confusion of IR with heat comes from.

ETA: Chronos covered this while I was typing.

Last edited by JWT Kottekoe; 01-08-2012 at 12:14 PM..
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  #16  
Old 01-08-2012, 12:15 PM
notsoheavyd3 notsoheavyd3 is offline
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Originally Posted by Chronos View Post
When you feel the heat from the Sun, that's mostly the visible light you're feeling anyway, not the infrared. Yes, infrared light can carry heat, but so can any light at all, via the exact same mechanism. The only reason people specifically associate infrared with heat is that most of the objects we're familiar with (animal bodies, fires, stove burners, old-fashioned light bulbs, etc.) happen to be at a cool enough temperature that most of their radiation is in the infrared range. But the Sun isn't "most objects", and is much hotter, hot enough that most of its radiation is in the visible range.

And since the OP said all light, it's easy to make a mirror that will work reasonably well in visible, IR, and near UV (in fact, most mirrors will work reasonably well in that range without even being specifically designed for it), but beyond that, not necessarily. So far as I know, there's no known way to make a gamma-ray mirror at all, for instance, and for far UV or X-rays, you usually have to settle for only a few percent reflectivity.
Don't forget radio Chronos. (I haven't checked a mirror but radio definitely goes through the walls of my house. Of course as we both know radio astronomy uses big mirrors that work in the radio range of the spectrum.)
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  #17  
Old 01-08-2012, 12:31 PM
johnpost johnpost is offline
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objects do reradiate light, they absorb shorter visible spectrum light and emit longer infrared light.
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  #18  
Old 01-08-2012, 12:38 PM
constanze constanze is offline
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Quote:
Originally Posted by Chronos View Post
When you feel the heat from the Sun, that's mostly the visible light you're feeling anyway, not the infrared. Yes, infrared light can carry heat, but so can any light at all, via the exact same mechanism. The only reason people specifically associate infrared with heat is that most of the objects we're familiar with (animal bodies, fires, stove burners, old-fashioned light bulbs, etc.) happen to be at a cool enough temperature that most of their radiation is in the infrared range. But the Sun isn't "most objects", and is much hotter, hot enough that most of its radiation is in the visible range.

And since the OP said all light, it's easy to make a mirror that will work reasonably well in visible, IR, and near UV (in fact, most mirrors will work reasonably well in that range without even being specifically designed for it), but beyond that, not necessarily. So far as I know, there's no known way to make a gamma-ray mirror at all, for instance, and for far UV or X-rays, you usually have to settle for only a few percent reflectivity.
So ... would Bob in the above scenario feel the heat on his back, via the reflection of the mirror, or would the general heat of a hot day be too much "ambient noise" and "drown out" the signal?

Nobody having sunlight at the moment, either?
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  #19  
Old 01-08-2012, 01:19 PM
notsoheavyd3 notsoheavyd3 is offline
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Originally Posted by johnpost View Post
objects do reradiate light, they absorb shorter visible spectrum light and emit longer infrared light.
At least for black body radiation the object reradiates ALL wavelengths of light. (So yes because of your body temperature you'll occasionally produce x-rays and gamma rays.)
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Old 01-08-2012, 01:22 PM
notsoheavyd3 notsoheavyd3 is offline
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Originally Posted by constanze View Post
So ... would Bob in the above scenario feel the heat on his back, via the reflection of the mirror, or would the general heat of a hot day be too much "ambient noise" and "drown out" the signal?

Nobody having sunlight at the moment, either?
It'd depend. It'd depend on how much light the mirror was catching(IE how big) and if it focused it or spread it out. (Or if it was a flat mirror so did neither.)
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  #21  
Old 01-08-2012, 01:24 PM
notsoheavyd3 notsoheavyd3 is offline
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Of course that being said I'd expect him to feel the heat since last I checked there used to be a huge solar energy plant in the American south west that used alot of mirrors to collect energy and shine it on a tower.

edit found ithttp://en.wikipedia.org/wiki/Concentrated_solar_power

Last edited by notsoheavyd3; 01-08-2012 at 01:25 PM..
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  #22  
Old 01-08-2012, 01:40 PM
astro astro is online now
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Originally Posted by Chronos View Post
And since the OP said all light, it's easy to make a mirror that will work reasonably well in visible, IR, and near UV (in fact, most mirrors will work reasonably well in that range without even being specifically designed for it), but beyond that, not necessarily. So far as I know, there's no known way to make a gamma-ray mirror at all, for instance, and for far UV or X-rays, you usually have to settle for only a few percent reflectivity.
This says gamma ray mirrors exist.

Quote:
Since they have the shortest wavelength of all EM radiation, gamma rays are extremely deeply penetrating.

Gamma ray mirrors do exist, but they are usually in the form of multilayer reflectors; sheets of reflecting material spaced by layers of gamma-transparent material. As a gamma ray passes down into the reflector, a succession partial reflections occur, one at each layer of reflecting material. A multilayer reflector is usually several metres thick, perhaps more, depending on the energy of the rays it works with.

Multilayer reflectors of this nature are much more effective at shallow, grazing angles. For a gamma ray approaching it at a higher angle, less energy will be reflected.

There are no theoretical limits to the efficiency of gamma ray mirrors, only engineering and economical concerns. The best attainable efficiency in practice probably peaks somewhere around 80% to 90%.
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  #23  
Old 01-08-2012, 01:40 PM
Lynn Bodoni Lynn Bodoni is offline
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When my daughter was curious about how a TV remote worked, I told her about infrared. Then how infrared was a form of light that humans can't see. She was about 8 or 10 at the time, and she'd also been curious about mirrors...so she pointed the remote at a mirror which reflected the image of the TV. Yep, the remote worked when its signal was bounced by the mirror. It wouldn't work when it was pointed that way, but not at a mirror.
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Old 01-08-2012, 01:42 PM
constanze constanze is offline
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Originally Posted by notsoheavyd3 View Post
It'd depend. It'd depend on how much light the mirror was catching(IE how big) and if it focused it or spread it out. (Or if it was a flat mirror so did neither.)
Flat mirror, because that's what average people have in their purses (at least I think). Alternativly a watch face, such as kids use to make circles onto the blackbord on a sunny day.
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Old 01-08-2012, 01:46 PM
notsoheavyd3 notsoheavyd3 is offline
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Originally Posted by astro View Post
This says gamma ray mirrors exist.
This link shows what they mean by a "mirror". (IE it doesn't so much as reflect as deflect)

http://astro.airynothing.com/2005/12/xray_mirrors.html
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  #26  
Old 01-08-2012, 01:47 PM
notsoheavyd3 notsoheavyd3 is offline
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Originally Posted by Lynn Bodoni View Post
When my daughter was curious about how a TV remote worked, I told her about infrared. Then how infrared was a form of light that humans can't see. She was about 8 or 10 at the time, and she'd also been curious about mirrors...so she pointed the remote at a mirror which reflected the image of the TV. Yep, the remote worked when its signal was bounced by the mirror. It wouldn't work when it was pointed that way, but not at a mirror.
You probably didn't know this at the time but the camera in a typical cell phone can see that light. (You could have just had her look at it with a cell phone camera and push one of the buttons.)
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Old 01-08-2012, 01:49 PM
notsoheavyd3 notsoheavyd3 is offline
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Originally Posted by constanze View Post
Flat mirror, because that's what average people have in their purses (at least I think). Alternativly a watch face, such as kids use to make circles onto the blackbord on a sunny day.
I'd expect a little mirror in a purse he'd feel because it should be big enough. Of course as long as it's directed at bare skin. The watch probably wouldn't work because it's actually round and would spread out the energy. (And it's smaller to boot.)
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  #28  
Old 01-08-2012, 01:53 PM
Musicat Musicat is offline
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Quote:
Originally Posted by Lynn Bodoni View Post
When my daughter was curious about how a TV remote worked, I told her about infrared. Then how infrared was a form of light that humans can't see. She was about 8 or 10 at the time, and she'd also been curious about mirrors...so she pointed the remote at a mirror which reflected the image of the TV. Yep, the remote worked when its signal was bounced by the mirror. It wouldn't work when it was pointed that way, but not at a mirror.
My guess is it's the hard surface, not the reflective layer, that bounces the remote signal. I have experimented with remotes bouncing off non-mirrored, not shiny, but hard, wooden walls, and they worked.
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  #29  
Old 01-08-2012, 01:56 PM
Chronos Chronos is online now
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Kudos to your daughter, Lynn! That shows both more scientific aptitude in general, and more understanding of mirrors, than the typical college freshman (and no, I'm not exaggerating).

astro, thanks for the information about gamma mirrors-- I hadn't heard of those before.

notsoheavyd3, with radio, the major issue isn't the material (pretty much anything conductive will work), but with the size: You have to have a mirror significantly larger than the wavelength.

And constanze, we're assuming that you can feel the radiant heat directly from the Sun, right? The only reason I can think of that the sunbather might not be able to feel it on his back, too, is that you'd probably have a smaller area illuminated. I'd still expect that he could, though.
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  #30  
Old 01-08-2012, 02:07 PM
constanze constanze is offline
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It's just that while I did reflect light circles of my watch face* onto the walls, I never experienced them on the skin, and neither other reflections of sunlight, so I'm unsure on whether the sunbather would feel it, or how much (depending on size, not quality of the mirror?)

* the cheap Quartz watches have flat glass faces, though
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  #31  
Old 01-08-2012, 02:19 PM
johnpost johnpost is offline
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Quote:
Originally Posted by Lynn Bodoni View Post
When my daughter was curious about how a TV remote worked, I told her about infrared. Then how infrared was a form of light that humans can't see. She was about 8 or 10 at the time, and she'd also been curious about mirrors...so she pointed the remote at a mirror which reflected the image of the TV. Yep, the remote worked when its signal was bounced by the mirror. It wouldn't work when it was pointed that way, but not at a mirror.
walls that are not dark also reflect enough light to work. quality remotes provide more than one LED so that there are many reflected paths and it is hard to not have it work.
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