Unbroken glass appears to be unique. But when it is joined at broken ends, it appears to be made up of 2 piece

Question

Unbroken glass appears to be unique. But when it is joined at broken ends, it appears to be made up of 2 piesces.Why it happens?

Answer 1

You mean uniform. The small air gap or glue at the join has a different index of refraction than glass. This causes some of the light passing through the glass to reflect, making the crack visible. There are special clear optical adhesives blended to provide a similiar index to glass. If glued with this, the crack becomes very indistinct.

Answer 2

A glass does not break exactly into two pieces. When we see it broken into two pieces, some very fine fragments of it are also lost. As a result when we bring the two pieces together, the distance between the molecules of one piece from those of the other piece are no longer as small as it was when the glass was in one piece. We, therefore, need to use an adhesive to join the pieces and they appear in two parts.

If the glass would have really broken into exactly two pieces, then by just bringing them together, the two pieces will join by force of molecular attraction and no adhesive would have been required and it will look exactly a single piece as earlier.