Why do some cracks in a glass show only from certain angles? I recently dropped my phone and, upon observation of the screen, I noticed something interesting with the cracks which formed. It seems so the cracks which I could see on the phone dependent on the angle I viewed the phone from. Could someone give an explanation for this happens?
 A: Actually, the cracks should be visible from all angles, but cracks will be most  visible depending on the angles of the incident light striking the cracks.
Light leaving the crack will do so at all angles because of reflection (and some refraction) of light coming off irregular formed cracks$^1$. But as you have stated, there are particular angles where the crack will be most visible.
When a crack in glass occurs, a gap of air forms between the cracks. This means there is a glass-air boundary in the region of the crack, and therefore total internal reflection will occur. But as stated this occurs at a particular angle, and this angle is known as the critical angle$^2$.
That is, light rays that are incident on such a glass-air boundary with an angle greater than this will not be transmitted into the glass, but reflected. Hence moving your screen around will show more pronounced cracks.
$^1$ It's actually due to a combination of irregular reflection of irregular surfaces, as well as total internal reflection and absorption in the glass.
$^2$ You can use Snell's law to trivially find that this angle $$\theta_c=\sin^{-1}(\frac{1.00}{1.55})\approx 40^\circ$$ where $1.00$ and $1.55$ are the refractive indices of air and glass respectively.
