What makes safety glasses safe?  I think this is a really good question.  Coupled with, how they differ from standard spectacles?

Firstly the material the lenses are made out of it very impact resistant, generally polycarbonate/phoenix or trivex, which undergoes ballistic testing.  To conform to Australian Standards for prescription safety glasses they even have to be a certain center thickness.

Standard spectacles historically were made from glass, which could be chemically hardened to make it harder to break but it wasn’t particularly safe.  More frequently lenses are made from standard plastic which isn’t particularly dense or impact resistant.  Don’t misunderstand me, wearing spectacles is more protective than wearing nothing unless your spectacles have glass lenses.  It is recommended for those of us with only one functioning eye that they wear spectacles to protect their good eye.   Though of course safety glasses would be even safer.

Once you start performing cleaning, painting, mining, construction or scientific testing or other jobs involving some risk to your eyes, wearing proper safety glasses is strongly advised.

Other than the lens material and lens thickness, the other main difference between safety glasses and standard glasses involves the side protection.  Either attached side shields or a large temple or a wrap-around lens that protect the eye from side impact.

Standard glasses do not tend to have the side of your eye protected.  They can also have large bridges across your nose – which is great for comfort but not good for safety.  Safety glasses tend to be snug around the nose to block things bouncing under the safety glasses between the frame and your nose and then hitting your eye.