blue mirror physics

I know the answer is No, I don’t know why

if you had a room with an ideal point source nonscattering monopigment light bulb (sold by GM in the year 2057 or something) as the only illumination of the room (meaning you had a light bulb that gave off one color only… say blue… just parallel lines radiateing from a point… an ideal blue light bulb, I doubt this abstraction effects it… but just adding it as a disclaimer so someone doesn’t answer right away "no because <something irelivent to the question>)

now say you had the room all lit useing this, could you use a peice of wood painted blue as a mirror? a colored ‘whatever’ is that color because it is reflecting that color of light, a mirror is a mirror because it reflects light… all light… (as does white things… I guess… huh?)

the way my girlfreind phrased this question: if you pointed a red pocket laser at a red square on the wall, would it reflect off? (odviously not, her question disproved my question…)

so whats up? why does mirror reflected light act more behaved than the light reflected to show color?

Hm, no.

An object absorbs the colour it is in.

The light would reflect off the blue wood. However, the wood has a rough surface, so a lot of light may reflect from it, but the original light rays’ directions will come off all mixed up. Any spatial correlations between light rays (i.e., any image present) would be lost. Thus: no blue wooden mirror.

Mirror-like reflections are often termed “specular”, while wood-type reflections are termed “diffuse”. In ASCII art:



specular:                              diffuse:
---------                              --------

light    light                         light   light
beam     beam                          beam    not-beam
in       out                           in      out
\\\      ///                           \\\  | /\\//|/
 \\\    ///                             \\\ \||/| /
  \\\  ///                               \\\ \\/ /_/
   \\\///                                 \\\/|\-/
 ==========  <--- smooth surface        ============  <---rough surface


(Not that the art helps – it was just fun to do.)

A given surface can have components of both types of reflections. (For example, some scissors blades.)

Also, a thick white sheet of copy paper probably reflects more of its incident visible light than your bathroom mirror (especially if you keep your mirror as clean as I do. :slight_smile: )

-P

A rough guide is that a surface will become a fair reflector when the surface features are smaller than the wavelength of incident light.

Assuming blue light has a wavlength of 0.5um, the surface should be smooth within 0.5um for it to give off a good, specular reflection. This of course assumes a reflective material.

RANGER, not sure if you mistyped, but a an object absorbs all the colors it is not. A blue object absorbs red and green and reflect blue. Blue sunglasses, absorb red and green and transmit blue.

I think you are confusing color and surface roughness. The sharpness of the reflected image is determined by the smoothness and flatness of the surface. The color is an inherent quality of the surface material and is independent of smoothness.

If you took a piece of aluminum and ground the surface with sandpaper, it would behave like a piece of white paper - it would reflect all colors (wavelength) at a pretty high efficiency, but at random directions. (This is also called scattering.) Now if you polished it with a very fine compound, it’d still reflect light at the same efficiency but now the reflected light is more ordered. So you get a sharp reflected image.

Now if you used gold instead, it’s yellow so it reflects red and green light but not blue light. If you grind it up you get a surface that scatters red and green light, and absorbs red light. If you polish it you get a mirror that reflects red and green, and absorbs red.

What? Scatters and absorbs red? What happens to the blue? What happens to the yellow?

I think someone is.

Sorry, that lshould have been “[reflects/scatters] red and green light, and absorbs blue” on both places.

Yellow is red+green. At least, that’s a good enough approximation for this discussion.

Pasta, that is one of the best ASCII visual aids I’ve ever seen in a Physics problem. It looks exactly how I picture scattered light. Good job. :slight_smile:

In a previous job I worked with IR imagers. It was amazing to see a piece of anodized aluminum turn into a perfect mirror when viewed with an IR imager.

This sounds like the flip-side of the old “what color is a mirror?” question that we’ve done a few times here. Can’t remember what the answer was, and too pooped to search.

flop

I’m not confuseing things… I was TRYING to phrase the question so people could answer what I was asking without ducking the question, the ideal light bulb and all… assume ideal ‘wood’ too, or ideal abstract monocolor surface.

would a blue mirror be possible in theory? if not in practice? a mirror that works for only a single color?

Obsolutely it is possible. Some filters work this way. They reflect the colors not wanted and pass the colors that are needed. IE, if you want red light to go through a filter, you could use thin films to reflect blue and green and pass red.

This idea is used extensively in the optics of lasers to weed out wavelengths that are not wanted.

Sorry. It wasn’t clear in the OP that was the question…at least not to me.

Mirrors that reflect one color and transmit others are called dichroic mirrors. They are used on some high-end video “3-CCD” cameras, where two dichroic mirros split incoming light into red, green and blue components. These are directed onto separate CCDs.

It’s easier to make a mirror that reflects one color and absorbs (rather than transmits) all other colors. You can make one yourself by covering a normal mirror with a color filter. If you use multilayer coatings you can even select a very narrow range of wavelengths to reflect.

Minor nitpick; let’s not confuse colour with wavelength; an object absorbs some wavelengths and reflects others; the reflected light may or may not be percieved as having colour by the human visual system.

Why am I saying this?

(In theory) An object could perfectly absorb all light of wavelengths that humans might describe as ‘yellow’, but the object will still appear yellow if it is simultaneously illuminated by (and reflects) wavelengths that humans might describe as ‘red’ and ‘green’ - A yellow object that would not be yellow in every sense of the word. (I’m not sure if such a material actually exists or could be made, but the point is - colour is an artifact of perception).