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My friend recently bought a high-powerd pocket laser pen (from China). It is green, and reflects off most surfaces as green.

On some orange surfaces, it reflects orange instead. That is to say, we see an orange dot instead of a green one.

It seems to be certain hard, shiny plastics (for example a Kryptonite bike lock). Orange fruit peel, for example, reflects green, and most orange fabrics reflect green.

I suspect it's some kind of pigment that is reflecting this way.

When the reflection is orange, the intensity of the reflection is subjectively about the same, i.e. it doesn't seem to be losing much energy. If one shined light from a green incandescent bulb onto an orange surface, one would expect that surface to be lit less intensely than a white surface. Not so the laser.

So it seems to be that the photons are being re-emitted at a different wavelength than they were absorbed at.

How can this happen? And why is it only orange?

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    $\begingroup$ This is an example of fluorescence. $\endgroup$
    – Peter Shor
    Commented Jul 28, 2013 at 21:56
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    $\begingroup$ When you say "we see an orange dot" - is that dot seen on the object being illuminated, or is the reflected light still collimated, and appearing as a dot elsewhere? It's an important distinction... because fluorescence is not coherent with the incoming light. $\endgroup$
    – Floris
    Commented Mar 4, 2016 at 4:28
  • $\begingroup$ The dot appears on the object being illuminated. $\endgroup$
    – spraff
    Commented Mar 4, 2016 at 9:37

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This is an example of fluorescence. Usually, when you shine a laser beam on a material and the photon is absorbed by an atom or molecule, a photon of the same frequency will be re-emitted. Sometimes, though, when a photon is absorbed, exciting an electron to a higher state, the electron doesn't decay to the ground state right away, but in several steps. In this case, when you shine light of one frequency on the material, it may emit light of a lower frequency. This is called fluorescence. Apparently whatever dye is using to color the orange plastic fluoresces orange when you shine green light of the right frequency on it.

There are lots of materials which fluoresce in the visible spectrum when UV light is shined on them. You don't see fluorescence with red lasers, because red is the lowest frequency of visible light, and photons emitted by fluorescence are almost always lower in frequency than the ones absorbed (there are apparently a few exceptions; see the wikipedia article). But it should be possible to see red, orange, or maybe yellow fluorescence with green lasers.

And here is a discussion of this phenomenon (with videos) on a blog.

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  • $\begingroup$ I think of the right frequency should be replaced by sufficient frequency. I bet that same material would also emit orange light when illuminated by a 405 nm laser. $\endgroup$
    – Ruslan
    Commented Oct 25, 2015 at 12:24
  • $\begingroup$ @Ruslan: That's not necessarily true. $\endgroup$
    – Peter Shor
    Commented Oct 25, 2015 at 12:26
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    $\begingroup$ In general this isn't necessarily true, but all fluorescent objects I tried, which were available at home, were of this type. $\endgroup$
    – Ruslan
    Commented Oct 25, 2015 at 12:27
  • $\begingroup$ @Ruslan: that's interesting. I wonder why that is. $\endgroup$
    – Peter Shor
    Commented Oct 25, 2015 at 12:30
  • $\begingroup$ I think their absorption spectrum has a wide bell shape in that frequency range. And after absorbing a photon, the subsequent electron relaxation leads finally to the same energy level(s), from which photon-generating recombination takes place. $\endgroup$
    – Ruslan
    Commented Oct 25, 2015 at 12:38
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You stated that the dot is orange so It is possible that photons from the laser are hitting electrons from the atoms which jump into an excited states. Once those electrons fall back to their original state they will get rid of their energy in the form of an photon with energy $W=h\nu = h\frac{c}{\lambda}$ where in your case $\lambda\approx 590–620nm$ (wavelengths are from Wiki).

I have remembered of the other possibility which could be some sort of a chemical reaction. Maybee ask on chemistry stackexchange.

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    $\begingroup$ I don't see how this answer makes sense. If an electron jumps into an excited state, and then falls back to the original state, why shouldn't it emit a photon of the same energy that it absorbed? Otherwise it will gain some energy, and won't be in its original state. $\endgroup$
    – Peter Shor
    Commented Jul 28, 2013 at 21:53
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    $\begingroup$ @PeterShor because there might be more than two levels involved, and only the "orange transition" generating a significant amount of light. This is just fluorescence. The point is that orange has lower photon energy than green (vice verse it would not work). Same as with UV light making white clothes appear glow white (and not emitting UV). $\endgroup$
    – Andreas H.
    Commented Jul 28, 2013 at 22:56
  • $\begingroup$ @Andreas: in fact, that's almost certainly what's happening, but this answer didn't mention that detail. $\endgroup$
    – Peter Shor
    Commented Jul 29, 2013 at 0:17
  • $\begingroup$ @PeterShor, true - the answer could be more detailed. But if it is really orange fluorescence, why should it be an orange substance (i.e. does not absorb orange)? Well it could be the case especially with several energy levels, but still I am not completely convinced. But maybe the OP did not mention all details, and I dont know an other physical effect that could cause this effect at laser pointer intensities. $\endgroup$
    – Andreas H.
    Commented Jul 29, 2013 at 0:38
  • $\begingroup$ @Andreas: Why shouldn't it be orange? If the substance absorbed orange light, it wouldn't be able to fluoresce orange very well, because the orange light emitted by the fluorescence would be absorbed. Of course, if it didn't absorb wavelengths in both the blue and orange regions of the spectrum, it wouldn't be orange, but apparently this isn't the case for this substance. $\endgroup$
    – Peter Shor
    Commented Aug 3, 2013 at 12:51

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