The opacity of glass cracks is related to the reflection, refraction and absorption of light by the glass. Cracks in the glass revealed rough edges.

When a beam of light hits this edge, it splits into multiple rays and reflects in multiple directions. These reflected rays interfere in ways that prevent a clear image from forming. This is called irregular reflection. Light will also enter the interior of the glass through the crack edge. Since the crack will cause the incident angle of some light to be greater than the critical angle, the light will not be refracted but will be reflected internally. This is also known as "total reflection". This makes the glass less transparent where it cracks.

Additionally, as light passes through glass, some of its energy is absorbed, causing the intensity of the refracted waves to decrease. The longer the distance that light travels in glass, the more light will be absorbed and the intensity of the emerging light will be weakened. Cracks in the glass will cause some light to be refracted and reflected multiple times inside the glass, causing the light to travel longer within the glass. This is one of the reasons why the cracks appear opaque.