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?
Yes, a mirror will reflect all light, visible + IR
Yes, both mirror and watch will reflect IR
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.
Yo** - it depends on the quality of the mirror whether it will reflect IR in addition to visible light
No - IR is too scattered/ short to begin with to be reflected by a normal mirror
… 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.
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.
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.
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?
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
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
Radio and Microwaves obviously depend on the type of material of mirror - see the dishes for radar, satellite etc.
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.)
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.
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.
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.
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.
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.)
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?
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.)
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.)