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?
Best Answer
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.