When I was lying on the beach after swimming the other day (on a rare hot and sunny day), i noticed that I still had drops of water on my skin in places. I also noticed that they seemed to be focusing the sun’s rays like a lens, which got me thinking - this can’t be good, right? I don’t recall ever hearing anything about this but it stands to reason that water drops might be able to focus UV rays down to create “hot spots” of skin damage.
Should we be worried about this?
I never got a spotted burn from water droplets. I have never observed droplet burns.
Based on my observations, water reflects and refracts some light (visible light), but I can’t speak for the UV part. However, lacking any droplet burns, I conclude that droplets do not do any 'magnifying". Maybe a random droplet has some odd physical properties that result in a concave shape that has the potential to focus and/or magnify the rays (doubt it).
As the first chart on this page shows, water strongly attenuates UV radiation (UV wavelengths are in the 10-400nm range). A droplet thick enough to do a decent job of focusing is thus probably passing only a small percentage of the UV it receives.
attenuation (in some contexts also called extinction) is the gradual loss in intensity through a medium
Well, it can be gradual or quite rapid, depending on the medium and the wavelength.
That chart is showing that the great majority of tanning rays WOULD make it through more than 1 cm of water.
The issue at hand is whether the droplets of water would magnify (focus) the rays increasing the risk of hot spots on the skin, potentially causing damage.
The answer seems to be “no”.
I disagree. The 10nm part of the chart is at the extreme left, where the absorption coefficient is really high and the penetration depth is really low. It’s true that at the longer UV wavelengths the absorption is dropping significantly, but these are the much less harmful frequencies.
Are the wavelengths that cause suntan/burn the same ones that darken photo grey glasses?
personal experience - If I have a drop of water on my photogrey eyeglasses, the glass behind the water does not darken nearly as much as the rest of the glass. I will have a pale spot.
So if it’s the same wavelengths, then the water seems to block more then it magnifies.