As light enters the glass, there is a refraction of light that is happening inside the glass. Refraction is where there is a change in phase velocity as the medium changes from the air to the glass. Now, when a glass is broken, the refracted light may be directed in the area where the crack. Since the light will be refracted in that direction, you will be able to see the light towards you at a suitable direction.
We can also relate this in cutting the diamonds. Manufacturers are doing some cuts in the diamonds that will make it sparkle brightly.