Why do most surfaces reflect laser light without changing the color?

[nhnifong]

I have a green laser. When I point it at most things, I see a green light reflected no matter what color the surface. Except when I point it at orange traffic cones, it reflects what appears to be monochromatic orange light.

Why do normal surfaces reflect light back in the frequency it came in?

Why do some surfaces reflect light back in some preferred frequency?

[Monkthespy]

Well i'm probably an idiot but this is how I think it works.

Mirrors are designed to reflect all light that comes in. So it will reflect green laser, because well, it's reflecting green.

The orange cone absorbs all colors except orange. So when you fire your green laser at it, it only reflects the orange value of the green, so you see orange reflected.

[nhnifong]

At least you admit that was a guess. :/

[lajoswinkler]

Some chemicals change the frequency of the photons that are transmitted/reflected, some don't. It has to do with the distribution of electrons in the molecules. Usually when there's lots of so called conjugated bonds

the spectra changes a lot.

When there's no change in frequency, only intensity is changed. Coal won't reflect much of it, snow will reflect almost everything, so coal in monochromatic 532 nm will look dark green, and snow will be shiny green.

Polychromatic radiation like regular light is a lot more complicated issue. Stuff has color because each frequency behaves differently, so the end result is synthesis of all that.

There are weird examples. Some solutions of chloroform are green on reflection and red on transmission. When you look at such liquid, you really can't tell what color is it. It depends on the angle for example.

Well i'm probably an idiot but this is how I think it works.

Mirrors are designed to reflect all light that comes in. So it will reflect green laser, because well, it's reflecting green.

The orange cone absorbs all colors except orange. So when you fire your green laser at it, it only reflects the orange value of the green, so you see orange reflected.

There is no orange in green lasers.

Edited September 18, 2013 by lajoswinkler

[nhnifong]

Thanks Lajoswinkler!

But where is the extra energy of the green light going when it turns into orange light? Are more orange photons emitted? It seems like this would leave atoms in an excited state because they have absorbed quanta of of 2.4 eV and re-emitted only about 2.03 eV.

Or does it mean the surface is also emitting 0.27 eV mid-infrared photons and I just can't see them...

[lajoswinkler]

Yes, this is basically fluorescence, except that the impacting photon is visible to us.

In some cases, even direct emission of infrared might happen, although indirect emission always follows up because of waste heat. One would have to filter the laser to ensure nothing except green is coming out, and then use an wide spectrum infrared camera to look at the point of impact, to check on that.

[Camacha]

Now this is an interesting story.

[K^2]

Just wanted to add that paints used for traffic signs or markers are very frequently fluorescent. It allows them to shine brighter in the headlights or whatnot. So it wouldn't be surprising if this was the case with the traffic cone.

But yes, most paints just absorb light, so they can't change the color of incoming monochromatic light, such as that from a laser.

[Pds314]

An orange cone is probably made of a fluorescent material, that is, it isn't really reflecting it as much as absorbing it and glowing it back out over a short span of time.

All other substances reflect most colors of light, and green is one of those colors, lasers are very specific as to what color they fire, some fire nothing outside a 100 picometer (0.2%) or less variation in color.

However, if you shine it at a black or magenta object, it shouldn't reflect hardly any light at all.

[softweir]

...

There are weird examples. Some solutions of chloroform are green on reflection and red on transmission. When you look at such liquid, you really can't tell what color is it.

Did you mean "chlorophyll"? I am quite certain that this is a characteristic of that famous plant pigment!