If glass is UV repellent, why do you get a tan through a window?


I just saw the new video by It’s Okay To Be Smart (https://youtu.be/FnDP1sjKGfU) and got confused when he said that glass is opaque to the UV light. If it is in fact UV light repellent, why are you still able to get tanned when exposed to the sunlight for example through a car side window?

In: Physics

I was always under the impression that you couldn’t. Am I incorrect in this? This would be interesting

Your car windshield protects from UV rays. The side windows provide half as much UV protection.

The glass is okay at providing UV protection. But the windshield is laminated with a layer of plastic in the middle (to prevent shattering). The plastic is very good at UV protection.

Not every window perfectly repels UV, also depends on where the sun is in the sky. When the sun is low, photons have to slug through more atmosphere to reach you, so if you’re trying to avoid a tan, stay inside during the middle of the day

The short answer is that your front windshield is generally considered to be around ~90%+ effective at blocking UVA and UVB rays, but your side windows are only about ~50% effective at blocking UVA rays which account for aging your skin (and tanning).

I am not a scientists or a physician but do work in medicine and have worked in the Dermatology specialty as a technician for 3 years.




UVA and UVB rays. Also, depending on Auto glass, some aren’t the same standard as others. My glass is both. My glasses stay normal. But when I get out, the side by the door is half dark.

It’s annoying.

Some glass is treated to absorb tons of UV, while other glass does allow a bit of UV through. You may get a tan after sitting next to a glass window for 4 hours, but you would get a similar tan out in direct sunlight without sunscreen in a few minutes.

Glass is at least partially transparent to UV light. The video did not give enough nuance. It is certainly opaque to infra-red light though.

Here is the best source I found: https://www.koppglass.com/blog/optical-properties-glass-how-light-and-glass-interact#:~:text=The%20color%20of%20a%20glass,appear%20blue%20to%20the%20eye.

UV is between 100-400 nm on the first graph and roughly the region 300-400 is allowed through. I have no iea what kind of glass this is though as you can greatly change its properties with the adition of other trace elements.

Here is a different source with a better graph on the third page (look at the dotted line). “https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/10/tie-35_transmittance_us.pdf

In cars, glass has been treated with extra layers of other stuff to absorb UV light to protect the occupants. This appears to mostly apply to the windshield.