Glass screen protectors supposedly protect your phone from impact. A youtube comment by McZidanne sums up this idea pretty clearly:
The thing about tempered glass cover isn't the protection they offer. It's the fact that they give in easier than the screen, so when a surge of energy is released into the phone as a result of a fall, the tempered glass absorbs that energy and protects the phone from having to absorb it.
However logically with my limited understanding of Physics this does not seem to be a phenomenon I can understand. A thin hard object shouldn't be able to reduce the force applied on the screen nearly at all, or can it? At least a plastic screen protector is soft, and can thus (theoretically) change the angle of the force partially (even if too insignificantly to be useful), but something hard as glass seems to me to be even less capable of protecting against impact. Am I missing something about the material properties of tempered glass?
Best Answer
To say that the glass "gives in easier" is a bit misleading, but this is the basic concept. To protect the real screen of the phone, one wants a protector that will absorb the shock from any impact, rather than that shock breaking the real screen of the phone. So, we want a material that is hard and tough so that it will withstand large impacts (i.e. dropping the phone from your hip on concrete). Tempered glass is harder and tougher than plastics, see figure 1 in the first citation below.
Yes, certainly! Appearances can be deceiving in nature, eh? For instance, a given weight of spider silk is stronger than steel!
Regular glass has about the same (see fig. 1) fracture toughness as commercial plastics. But tempered glass has a much higher fracture toughness than regular glass (that's the whole point of it), especially if they're chemically tempered rather than thermally tempered. Note that glasses generally have much larger hardness than plastics as well, so toughness is the dominant contributor to the material's ability to survive a fall, no matter how it falls.
So, tempered glass has a larger resistance to fracturing (it is tougher) than plastic, meaning that the tempered glass can absorb more energy from an impact than the plastic. Thus, for a given impact force, the tempered glass is less likely to break than the plastic.