Technology
Glass is never quite transparent.
Some of the light that reaches it comes back.
It is always there, on the cleanest glass, in daylight and after dark.
None of it is a fault in the glass. A little of the light simply turns back at the surface — and always the same small amount, whatever the glass is made of or how well.
Which is the more interesting question: why always the same amount?
Figure 01
Light moves easily through air, and easily through glass. What it cannot do is pass from one into the other as though the two were the same.
At the surface — the thin line where air becomes glass — a fixed share of the light turns back. That share is set by the meeting of the two, not by the glass. Wherever two clear materials meet, this is what light does.
Figure 02
The surface cannot be removed, and it does not need to be. What can change is how the light arrives at it.
A layer thinner than anything you could see, laid over the glass, is enough to alter that meeting. The reflection does not vanish. It grows quieter, and a little more of the light comes through.
You cannot stop the light turning back. You can only persuade less of it to.
Figure 03