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Only UVA rays can pass through glass, so you won’t get sunburnt sitting by a window, while you can still enjoy the warmth of the sun. Be careful when sitting in the shade, however. You can still get burnt because you will be exposed to rays bouncing off reflective surfaces nearby. This is particularly the case near water. Not all clothing offers effective protection, either. If you can see the light through a piece of clothing when you hold it up, it will not offer much of a barrier to UV rays.