I'll say it in simple language, write in your own words okay.
When we are looking at the window, we are basically perpendicular to it.
when the rays of the sun hit the window surface at SOME PARTICULAR ANGLE, which is its critical angle.. the rays after refraction exit the glass with the angle of refraction as 90°.. that is parallel to its surface.
Now, where these cracks are formed, there are 2 new "surfaces" formed,
the refracted rays go parallel to these surface and reach us, because we are standing perpendicularly.. visualize it..
when these refracted rays fall in your eyes, we see it as silvery.