I was perusing Wikipedia the other day, when I read the article on fluorescein. The article claims that the molecule is often used as a dye for rivers. I had heard this, too. It makes sense, because fluorescein is generally considered nontoxic and it’s very fluorescent, so it would be a good marker.
But something puzzled me: why is the Chicago river green in the image? When I think fluorescein, I think “Absorbs at 487 nm and emits in the green (510 nm).” Fluorescein in water should look yellow. In fact, there is an image in the Wikipedia article of an eyedropper with a yellow fluorescein solution. And here’s a pic of my vial of fluorescein in water:
Sure looks yellow to me.
I mentioned my confusion to a friend. We concluded that they must dump a shit-load of fluorescein into the river to make it look green. He decided to test this by putting some fluorescein in water (I’m sure he’s not pouring that stuff down the drain!):
As you can clearly see, water looks green. And he didn’t even use very much dye (probably ~1 mg).
Here’s the point: when you look through the solution in a vial, it looks yellow because you’re seeing the transmission. When you look at the solution in a river (or a ice bucket) from above, you only see the fluorescence excited by sunlight. You can even see that it looks more yellow as he pours it out of the ice bucket (because then you can see some transmission as well as fluorescence).
Well, duh! Now, I feel silly for being confused: that was so obvious. I forgot that the quantum yield of fluorescein is nearly unity. Thanks, Wikipedia.