system
Given the following post, respond as if you're commenting on Reddit:
human
How thick does a piece of glass have to be before light cannot travel through it?

I was looking at a window today, and realised that the window must block some light, so I was wondering how thick a piece of glass would have to be before no light would make it from one end to the other.
gpt
The best (clearest) glass is the high-quality fused silica inside the optical fibers used for telecommunications.  These are amazingly clear -- they attenuate light (at 1.5 micron wavelength -- a wavelength the human eye can't see) at a rate of 0.2 dB/km.  At that rate, considering a reasonably bright light (1 mW) and the fact that we can usually see light about a factor of a million weaker (about 1 nW), light can travel 300 km (190 mi) before you wouldn't be able to see it any more.  At the red end of the visible spectrum where the attenuation is around 5 dB/km, if you pointed a red laser pointer in one end of the fiber, you'd still be able to see it after about 12 km (7.5 mi)!