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On this winter solstice, as we anticipate the return of light, I thought I'd write about a different kind of light.

Merriam-Webster defines fluorescence as "luminescence that is caused by the absorption of radiation at one wavelength followed by nearly immediate reradiation usually at a different wavelength and that ceases almost at once when the incident radiation stops". It is a type of luminescence that occurs in both biological and non-biological objects. For example, mushrooms and scorpions are notably fluorescent, as are several minerals. Technically, to qualify as "fluorescent" an object can absorb any wavelength of radiation and re-radiate any other, although the re-radiated wavelength is usually longer than the absorbed wavelength.

We humans, with our three (and occasionally four) color photoreceptor types, can see only the tiny fraction of the electromagnetic spectrum that we call visible light. The visible light range (approximately 400-700nm) is bounded by UV on the short end and infrared on the long end. Other organisms have very different light perception capabilities. We know, for example, that insects can see in UV and pit vipers can see in infrared. And as for mantis shrimps, which have as many as 12 types of photoreceptors, we don't yet understand how they see the world, but you can bet it's nothing like the way we do. For practical purposes, fluorescence is most easily seen when an object absorbs UV light and re-radiates light of a longer wavelength that falls into the visible light range.

When you shine a UV light on one of these fluorescent objects, you see an apparent color change from whatever it looked like under visible light. This color change is most striking in the dark, because the fluorescent object will appear to glow. The same thing happens in daylight, but is obviously more difficult to see.

Here, let me show you. A few weeks ago I went to Natural Bridges to photograph the anemones, first under normal daylight conditions and then under UV light. I have a pretty wimpy UV flashlight, it turns out, but you can still see the fluorescence.

Here's Anemone #1, under daylight:

Sea anemone in daylight
Sunburst anemone #1 (Anthopleura sola) at Natural Bridges
2021-12-07
© Allison J. Gong

And here's Anemone #1 under UV light:

Sea anemone under UV light
Sunburst anemone #1 (Anthopleura sola) at Natural Bridges, under weak UV light
2021-12-07
© Allison J. Gong

Striking difference, isn't it?

This is Anemone #2. It was getting dark by then, but this photo was also taken without flash and I did not increase exposure of the image.

Sea anemone
Sunburst anemone #2 (Anthopleura sola) at Natural Bridges
2021-12-07
© Allison J. Gong

And, under UV light:

Sea anemone under UV light
Sunburst anemone #2 (Anthopleura sola) at Natural Bridges, under weak UV light
2021-12-07
© Allison J. Gong

Here's what's going on. Pigment molecules in the anemones' tissues are absorbing the UV radiation and re-radiating light in the visible range. It's easier to see the fluorescence in Anemone #2 because it was much darker when I took that set of photos. Fluorescence still occurs during the day, but we can't see it as well in the daylight. This is why our local bowling alley does their Atomic Bowling at night! They can dim the overhead lights, crank up the black lights, and let the tunes roll.

Incidentally, if you've ever wondered why so-called black lights are purple, there's a reason for it. A true black light emits only UV light. UV light is invisible to us, hence the term "black", as in pure darkness. UV lights that ordinary folks like us can buy are tinged purple so that we can see it. The purple isn't UV, of course, but seeing the purple light keeps people from looking into the beam and frying their retinas from the actual UV radiation.

Sea anemones, of course, do not celebrate the solstice, but they do perceive it. They, and just about every other living thing, can sense the cyclical changes in day length as the year progresses. After tonight the days will start getting longer as we move through winter and towards spring. Personally, I cannot wait until we get the early morning low tides in the spring.

In the meantime, happy solstice, everyone!

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