One of our readers asked us this question.

Reading “Surely You’re Joking, Mr. Feynman!” he mentions sitting in a truck watching the atom bomb test through the window because UV light cannot pass through glass – he also says bright light can never hurt your eyes, the real danger is UV. But, it seems like I have had my arm get slightly sunburned while driving long distance on bright days with the sun directly on the driver’s side. So am I hallucinating or can one get burnt through glass and does thickness matter at all? How would a UV light let any UV out if its tube is made of thin glass?

Short answer: Glass doesn’t block all UV rays. UV radiation comes in two types, UVA and UVB. UVB is responsible for the most intense burning, and is blocked by untreated glass. UVA penetrates deeper into your skin and is responsible for tanning, and also penetrates untreated glass.

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Long answer: As a general rule, don’t take medical advice from Feynman. He did crazy things, like look at the first nuclear detonation on earth without safety equipment.

Now that that’s out of the way, I should inform you that we subdivided the UV part of the spectrum into UVA and UVB, with respective wavelengths of 400-320 nm and 320-290 nm.

At the earth’s surface we receive vastly different amounts of UVA and UVB rays; UVA accounts for about 95% of them and are present all year because they penetrate the ozone layer. UVB rays, which are slightly more energetic, are partially shielded by the ozone layer, so they are more abundant in the summer.

Furthermore, UVA penetrates most glass, and penetrates deeper into your skin, and is responsible for tanning (and also a lot of aging as well). While UVB does do most of the burning it is generally blocked by glass.

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So, while a hypothetical person might prevent the nastiest of the sunburn by hiding under glass, they’ll still be soaking up a lot of UV rays. I don’t want to inadvertently give medical advice here, but I don’t think anyone will fault me for saying, “Full spectrum sun screen (UVA+UVB) provides greater skin protection from sunlight than glass.”

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Last Update: September 4, 2021

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