Protecting your eyes (along with hearing) is one of the most important safety precautions you can take. Whether you are mowing the lawn or working in a shop, you must protect your vision. There are standards set by the American National Standards Institute for safety equipment to protect your vision.
The standard set by American Standards National Institute for prescription safety glasses is known as ANSI Z87.1-2010. This creates and sets testing and performance requirements for eye protection devices (prescription and plano). This includes prescription and non-prescription (plano) safety glasses, welding helmets, face shields and goggles.
The ANSI Z87.1-2010 standard also contains testing criteria, descriptions and basic requirements for the use, care, marking of and selection of the protective equipment.
The ANSI Z87-2 (prescription safety glasses) must be clearly marked on the frames of prescription safety glasses along with the manufacturer name. If the lenses are high impact rated then they must be marked Z87+ or Z87 if not high impact rated. If the glasses have side shields they must display the manufacturer name and the Z87 mark or Z87+ if impact rated.
Lens may be made from polycarbonate. Lenses made form polycarbonate are extremely light weight and virtually shatter proof.
Here is an inexpensive pair on Amazon with polycarbonate lenses and bifocals. You can even choose the prescription strength.