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This morning it was particularly foggy and my son was excitedly looking at the sun through the fog, since of course he normally can't look directly at it and see the disk. I told him to stop looking directly at it and said that the fog blocked some wavelengths but not others, and so it could still be dangerous.

Given that he's 7 years old, I'm sure it was the right advice from a parenting perspective (we can worry about nuance when he's older, and fogs can shift quickly). But was it correct from a physics perspective? Does a dense fog allow through invisible wavelengths (which might be harmful to the retina), or does it block other wavelengths the same as visible light?

David Z
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T.J. Crowder
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  • According to a dermatologist I once asked, clouds (and by extension, fog) only attenuate UV light by about 10%. Without seeing a graph of EMF attenuation by wavelength I don't know who I would believe. Of course, attenuation is not the only factor - scattering could reduce intensity from the sun while spreading it across a wider area. I recall the old adage of wearing sunblock when it is cloudy... – Michael Sep 21 '15 at 02:04
  • @Michael Your dermatologist might be right. Note that fog makes light come from all directions instead of straight from the sun, which is just as bad for your skin. The reason that a 'clear' sun is a problem for your eyes, is because your eyes focus the sunlight into one spot on the retina. – Sanchises Sep 21 '15 at 08:22
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    @Michael That would be the graph in my answer then... – ProfRob Sep 24 '15 at 08:22

3 Answers3

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The damage to the eye from looking at the Sun is thought to be due to high intensity light creating free radicals that attack the cells in the retina. Contrary to popular belief it isn't a simple burning process.

To a first process fog attenuates all (visible) wavelengths equally, which is why it's white. If it preferentially absorbed some wavelengths it would cause a colour change. If you're interested in the details a quick Google found this article on light attenuation by the various forms of fag, cloud etc.

So fog doesn't let through potentially harmful wavelengths whilst blocking others. It blocks everything.

Nevertheless I strongly approve of your advice to your son - looking at the Sun is a bad habit to get into!

John Rennie
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    Thanks, very useful! I figured the fog could be blocking light in the visual spectrum without blocking light outside it (thus the fog could be white, to my eyes, even when preferentially blocking some wavelengths over others). Are you saying it doesn't do that? Or that while it may, the light that can cause the damage is also the light we can see, so... :-) – T.J. Crowder Sep 20 '15 at 08:12
  • I can't fully agree with the notion that fog is a good grey filter. Fog does let infrared wavelengths trough, which is why infrared photography is used by the military. Fig S1 of the document you link to clearly shows that fog does not attenuate equally well in the infrared as it does in the visible range of the spectrum. Having said that, the attenuation is more than likely enough to make viewing of the sun harmless. And having said that I would also strongly discourage risky behavior with ones most sensitive organs. – CuriousOne Sep 20 '15 at 08:13
  • @CuriousOne: Infrared was indeed in my mind... – T.J. Crowder Sep 20 '15 at 08:17
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    @T.J.Crowder: as CuriousOne says, fog will let through infra-red light. However that has a far lower intensity that the visible wavelengths. The spectrum of sunlight peaks around 500nm (blue-green). So the IR isn't going to be any danger. – John Rennie Sep 20 '15 at 08:17
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    @T.J.Crowder: I agree with John Rennie: only a small fraction of the sun's power is in the IR light that will pass the fog. Those wavelengths will not cause sunburn. The heating of the retina is more than likely not a serious problem, but I would still not encourage viewing of the sun under any circumstances for more than a few seconds. My rule for laser safety is this: a physicist has two experiments that involve lasers and eyes, they are called "left eye" and "right eye". I would look at direct sun exposure the same way. I did have a light retinal burn once... it hurts bad! – CuriousOne Sep 20 '15 at 08:22
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    I was about to say that IR wouldn't reach the retina anyway, but somewhat to my surprise the eye is quite tranparent to near IR. I found this paper on the subject with this graph of the transmission through the eye. – John Rennie Sep 20 '15 at 08:22
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    Not wishing to draw things out, but having largely dismissed IR, what do we think of UV? – T.J. Crowder Sep 20 '15 at 08:23
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    There may be another component to this: if the eye is not exposed to much visible light, the iris might not be fully closed, so there may be much more IR reaching the retina than it would in case of a full retinal reflex. That may make things worse by probably an order of magnitude. – CuriousOne Sep 20 '15 at 08:24
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    @T.J.Crowder: my guess is that uv is strongly scattered by fog/cloud. Annoyingly the measurements in the paper I linked stop around 400nm, which is about the short wavelength limit of human vision, and uv is below this at around 250-400nm. I'm sure Googling will find you articles on uv scattering by fog, but as always caveat Googler. – John Rennie Sep 20 '15 at 08:29
  • @T.J.Crowder I think UV light is the primary concern and fog lets it through pretty much to the same extent as longer wavelengths (except IR). See my answer. – ProfRob Sep 20 '15 at 09:21
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I think that from a medical perspective your advice was correct, but your physical explanation of why was not.

The wavelength dependence of the extinction due to fog depends on the distribution of particle sizes. If the particles are bigger than the wavelength of light, then the (Mie) scattering and extinction become independent of wavelength, and this I think is the usual case - Price (2010) provide a brief review and some data for fogs and the consensus seems to be that typical droplet sizes are 10-20 microns, though can be bigger or smaller depending on exactly what type of fog/mist it is. Grabner & Kvicera (2011) discuss this in some detail and show (model) curves of extinction versus wavelength that are quite flat across the optical and UV ranges (though do allow more IR light through) unless the fog particle size falls below the wavelength of light.

Extinction due to fog

Now, if you think that the problem with the Sun is due to UV light entering the eye, then the effect of fog would simply be to allow you to expose yourself to more UV light, as the attenuation at other wavelengths would also stop the reflex you would normally have to blink or turn your eyes away or for your pupils to narrow.

Thus although there may not be any acute damage caused by looking at the Sun through fog, you would certainly be increasing your long term exposure to short wavelength radiation if you got into the habit of doing it, possibly leading to cataracts and macular degeneration. The risk is thought to be cumulative. A brief look at some sources suggests that recommended sunglasses block much more (or even all) UVA and UVB radiation (290-400nm) compared to longer wavelength radiation for this very reason. Fog is like "grey" sunglasses, allowing you to damage your eyes without the acute sensations which tell you that damage may be occurring

Absent a contrary medical opinion, I would say that UV light from the Sun is particularly harmful and I would not stare at the Sun through fog.

ProfRob
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  • So basically, if you want to be really safe look at the sun through fog while wearing UV blocking sunglasses –  Sep 24 '15 at 09:16
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To add to the other answers: without detailed analysis of the light passing through fog, one cannot infer that, just because fog dims the Sun to a level that makes staring at it comfortable, therefore it is safe.

The "brightness" of the Sun, and the discomfort that staring at it induces, is only very weakly related to the damage it can do. Indeed, a healthy retina can easily handle the heating effect that staring at the Sun at high noon will impose on the retina. What makes the light dangerous is its UV content, which damages the retina through phototoxicity, not heating damage. And very low levels of UV can be damaging, especially for little children. The eye's lens attenuates UV significantly by the age of 20, but in children it is greatly more transparent to harmful UV.

The reason comfort in looking at the Sun is not a good guide to safety is because staring at it induces temporary blindness (that rights itself after a few minutes) - not a good situation for prey or predator animals (we are both), so it is a strong evolutionary driver to evolve a blink / aversion response. But this is not the reason sunlight is a concern now: the effects of Sun UV - cataracts and retinal cancer - usually show themselves too late in life for them to be an evolutionary driver (our forebears were mostly dead from being eaten or whatnot long before these diseases showed themselves).

See my answer here which gives some more info and also cites some gruesome animal experimentation illustrating the above.

Community
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Selene Routley
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  • Thanks. My instinct, as you saw, was indeed that "just because we can look at it without pain that doesn't mean it's safe". :-) – T.J. Crowder Sep 20 '15 at 11:08
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    @T.J.Crowder Indeed. Watch out for Neogene lions wearing sunglasses, lying in wait to pick off he-man Neogene hunters playing who can stare at the Sun the longest in their lunch break. – Selene Routley Sep 20 '15 at 12:08