Jennifer, to completely answer your question, we need to know a little about the sun & a little about glass properties.
Let start with the sun. We all know that the sun is the cause of the sunburn but we should be aware that light coming from the sun, even though it looks yellow, contains many colors (many wavelenght).
In fact the sun emits a spectrum of color (wavelenght) & we see it yellow simply because it emits more yellow than red & blue for exemple.
Now we have to also note that, along with red, yellow & blue, the sun emits radiation that is not visible & part of it is called ultraviolet (UV). We cannot see this color because our eyes are not made to detect it but the sun emits plenty of UV. In fact this UV light is so strong & energetic that it burns our skin hence the sunburn.
Now about the glass.
Glass is transparent because it lets all the colors go true it but what we can not see is that it acts as a filter for UV light. Most of UV radiation is stopped by glass & this is why you will not get sunburns behind a glass.
The glass simply filters out the UV radiation that is responsible for the sunburns & protect your skins from these energetic & somewhat harmful radiation. Sunblock lotion works exactly the same way, they simply filter out UV light.
Gilles Lalancette, M.S., Physicist, Dorval, Qu'bec, Canada
You can, actually; it depends on the glass.
We know that electrons in any molecule can only absorb radiation at certain frequencies. It turns out that the electrons attached to molecules in typical glass (like the glass in your windows at home, or the safety glass in car windows) can absorb radiation at UV wavelengths, but not at visible light wavelengths ' therefore, visible light passes through glass as if it weren't there, but UV radiation is absorbed. It depends on the glass exactly how much UV radiation is absorbed, though. UVB rays are shorter than UVA rays -- that means they're more energetic, and they're usually the ones responsible for a sunburn. UVA rays are closer to the visible part of the spectrum, so it makes sense that some UVA radiation can make it through the glass. There is a difference of opinion on exactly how much damage UVA radiation does to you, but we do know that since it has a longer wavelength, it goes deeper into your skin. While UVB rays seem to be the primary culprit in skin cancer, UVA rays have been implicated as well. And according to NASA, most tanning salons use UVA radiation, probably because it burns less but still causes the pigmentation in skin that manifests as a tan.
The upshot, though, is that glass should shield you from most of the harmful UV radiation running around out there.
Rebekah Cowdrey, Physics Undergrad Student, U. St. Thomas, St. Paul
'If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.'