Question

why does a crack in a window pane appear silvery at some particular angle?

Answer 1

 
crack in a window pane appear silvery at some particular angle because of following reasons:
1. There is always an amount of  air present in the crack.
2. When rays of light travelling through glass, strike the glass air interface at an angle, greater than critical angle of glass, they are totally reflected.
3.When these reflected rays reach our eyes , then to eyes they appear to come from the crack, which in turns appears to be silvery.