I have some green, blue, and purple markers which clearly fluoresce under monochromatic light with a lower wavelength. Here they are illuminated by 589nm light emitted by a low pressure-sodium lamp (SOX light):
And here lit by the diffuse light from a a 532nm +/- 10 green laser:
This is surprising because fluorescence is defined as:
the visible or invisible radiation produced from certain substances as a result of incident radiation of a shorter wavelength such as X-rays or ultraviolet light.
By my understanding, that means it shouldn't be possible for either the blue or purple marker to be fluorescing under either light source because blue and purple light has a shorter wavelength.
What is happening?
More Photos
Markers lit by 650nm +/- 10 red laser light. The blue marker looks like it might be slightly blue to my eye, but it's hard to say.
Markers lit by incandescent light:
Low Pressure Sodium Lamps
The yellow light is a low-pressure sodium lamp (also known as a SOX lamp). As wikipedia describes:
These lamps produce a virtually monochromatic light averaging a 589.3 nm wavelength (actually two dominant spectral lines very close together at 589.0 and 589.6 nm). The colors of objects illuminated by only this narrow bandwidth are difficult to distinguish.
Viewing the SOX lamp through diffraction grating with 100 lines/mm:
In contrast to an incandescent light viewed through the same grating:
In contrast, "high-pressure sodium lamps emit a broader spectrum of light than the low-pressure lamps, but they still have poorer color rendering than other types of lamps" per wikipedia. High pressure sodium lamps are more common in streetlights.
The green laser shined at the wall through the diffraction grating:
Unlikely Explanations
Both prior research and answers/comments here make these seem like unlikely explanations. I have retained them for context.
I found a chemistry question here which explains that:
[Emitting light at a shorter wavelength] is certainly possible to make in a fluorescent / phosphorescent light scenario, but the engineering challenges (or, more specifically - the cost) involved in such a solution would be steep.
Another answer notes:
This method is called two-photon microscopy. Two photons of longer wavelength are absorbed by a dye molecule which then emits one photon of shorter wavelength. As mentioned before, this process is highly unlikely, but this property can be used to reduce the size of the fluorescent spot in confocal mocroscopy.
These answers provide a mechanism by which emission of green, blue, and purple light is possible, but make it sound like that only happens in exotic, expensive materials. Noting the process is "highly unlikely" also makes me expect that the blue and purple fluorescence would be much dimmer than they are.
These markers were bought in Thailand for just a couple dollars. While they glow the brightest of any green-purple objects I have, they are far from the only thing I own that exhibits this phenomenon.