Do I have to provide vari focal or bi-focal safety glasses for our employees or can I just supply overglasses

As David bannister said this approach is probably with the letter of the law but probably not the spirit. We tend to restrict issue of over-glasses to visitors and people working in our labs for short periods, 3 months as a rough guide (eg work experience). Anybody else who needs them are offered prescription safety glasses. One thing we have noticed is that if they have their own prescription glasses they tend to wear them and look after them better. Over glasses tend to get lost and broken, so are really a false economy.

People get bifocals because their eyes cannot accommodate distance and close up vision with only one prescription. So if you don't get their safety glasses made up like this, you have to choose between them not being able to do close up work, or not being able to see into the distance! You might be able to argue that a bifocal is an acceptable alternative to a varifocal, but to argue that a single lens would be adequate might be pushing it. Speaking for myself I graduated from a single prescription to bi-focals (reluctantly) when I found my arms were no longer long enough to be able to read. Now I have graduated to only requiring lenses for close work, as my distance vision has slowly improved as I have become more wrinkly!

I tried bifocals but switched years ago to varifocals. With a bifocal you either have a lense that is for close reading or a lens for distance vision. I found this a problem when working on a computer as neither lens gave the correct values for viewing the computer screen. Modern varifocal lenses have a transition zone, so that I can use the lower part of the lens for reading/close work, the central part for the computer and the top part for distance. So if you have people who need all three the varifocal might be a sensible answer and avoid them having to remove the bifocals to work on the computer, with the risk that they then don't put them back on.