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Someone recently told me that windows act as a kind of "magnifying glass". That doesn't make a lot of sense to me. Here's an example of someone with that belief (in the comments):

Can a baby get sunburned while laying in the sun through a window

Here's another example (about car windows):

Do glass car windows increase the strength of uv rays?

But it seems to me that regular (flat) windows shouldn't behave this way. So that leaves us with regular UVA/UVB.

According to Wikipedia:

"Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths, whereas silica or quartz glass, depending on quality, can be transparent eVen to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm."

As I understand it, UVB are more dangerous than UVA, but what if you have a very sensitive skin and stay several hours behind a glass window? Can you still get a sunburn?

Oddthinking
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Olivier Bruchez
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    I have very fair skin, as I'm a red head. I've been sunburned through my car windows numerous times. However, I could probably get burned through a brick wall. – George Feb 29 '12 at 13:48
  • "Somebody told me" does not make this claim notable. Can you please provide [any example](http://meta.skeptics.stackexchange.com/questions/1491/what-is-notability-and-how-should-we-enforce-it) of people making this claim? Thanks – Sklivvz Feb 29 '12 at 23:31
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    FWIW: I have heard the claim (from my father) that glass *blocks* UV and hence you *cannot* get sunburnt through windows. (Seems clearly false in the face of evidence to the contrary.) – Oddthinking Mar 01 '12 at 01:58
  • Depends on the glass and what is is treated with. When I had [Photochromic lenses](http://en.wikipedia.org/wiki/Photochromic_lens) they absolutely did NOT get dark inside of my car but did standing in front of an old glass door. – Sam I Am Mar 01 '12 at 02:46
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    @Oddthinking: glass blocks certain UV wavelength, and not necessarily at 100%. See references in my comment to the answer. – nico Mar 01 '12 at 06:50
  • @Sklivvz: I've added a few links mentioning people with that belief (magnifying glass effect). – Olivier Bruchez Mar 01 '12 at 08:27
  • My wife works as traffic teacher and spends much of the work day in car. She gets a lot more tanning on her arm that sits closer to the window, but she don't get a burn. She burns very easily. Since this is not based on public references, I just post it as a comment - not an answer. – awe Mar 01 '12 at 14:05
  • Toward the "notable claim" requirement, in one of Feynman's autobiographies he tells of being the only person to observe the trinity nuclear bomb test with "bare" eyes, having reasoned that UV was the threat and that glass block UV and accordingly gotten into a truck to watch through the windscreen. The answer of course is that exactly what wavelengths are blocked a and how well depends on the glass. – dmckee --- ex-moderator kitten Mar 22 '12 at 20:43

1 Answers1

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This answer addressed the question about whether you can get a sunburn trough a glass window. This is what the title and the last paragraph of the question mentions. The "magnifying glass" part doesn't really fit with that, IMHO, and could be a different questions by itself.

According to Can glass block sun rays that cause skin cancer? (cancer research UK) normal glass windows reduce the UVB part (wavelength range 315–280 nm [1, 2]) but not the UVA part (400–315 nm, [1, 2]):

UVA causes skin ageing and research has now shown that it is also likely to cause skin cancer. UVB causes redness and sunburn and is a major risk factor for all types of skin cancer.

Most glass used for windows blocks UVB but not UVA. This means that although glass might reduce the risk of sunburn, it does not prevent long term damage from UVA.

Important here is the part which states that UVB causes sunburn. This is also stated by:

http://www.skincancer.org/prevention/uva-and-uvb/understanding-uva-and-uvb

UVB, the chief cause of skin reddening and sunburn, tends to damage the skin's more superficial epidermal layers.

and Wikipedia:

http://en.wikipedia.org/wiki/Sunburn:

Ultraviolet B (UVB) radiation causes dangerous sunburns and increases the risk of two types of skin cancer [..]

Further sources for the UVB blocking by glass windows are:

http://www.smartskincare.com/skinprotection/uv-indoors.html:

Most people believe that ordinary window glass blocks UV radiation, making any other UV protection unnecessary indoors. This is half-truth at best. Window glass blocks UVB but lets much of UVA through.

UV exposure in cars (research paper):

UV wavelengths longer than >335 nm were transmitted through car windows, and UV irradiation >380 nm was transmitted through compound glass windscreens

(This means they both block UVB (315–280 nm) completely.)

Photoprotection by window glass, automobile glass, and sunglasses (research paper):

It has been known for some time that window glass filters out UVB and transmits UVA and visible light.

Note that this paper mentions that there is also special glass which blocks the whole UV spectrum including UV-A, but normal glass doesn't.

These bring me to the following conclusions:

  1. The risk of sunburn through a glass window is, depending on the source, either heavily reduced or not existing compared to direct exposure to sunlight. In the first case you could still get a sunburn after a longer time.
  2. Because UVA is not blocked at the same time the use of sunscreen is still required in order to avoid skin damage like skin cancer. This is a real danger, because the warning effect of the sunburn is then missing or delayed.
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Martin Scharrer
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  • Actually, normal glass won't block UVA much, but many types (like windshield's or eyeglasses') are treated to block it. – Quora Feans Apr 18 '17 at 10:08