Is it simply the amount of visible light entering the eye? I'm simply confused as to why experts say that sunglasses do not provide adequate protection when they are advertised as blocking 100% of the sun's harmful rays.
The damage to your eye does not only come from UV exposure, but also from infrared (IR) radiation. Your eye contains a lens that focuses the incoming rays to a narrow point. This point would get very hot if you looked directly into the sun. This is comparable to focussing the sun on a piece of paper with a magnifying glass, it would start to burn. Also, the rods and cones in your retina are only tolerant to a certain exposure of visible light.
Visible light too is dangerous to have concentrated into your retina.
When you shine a flashlight at yourself, where does all that visible light go as it seemingly disappears into your body?
It makes your molecules more ever so slightly more.
Also, a lot (but not all) of the uv light is absorbed by your cornea.
Visible light will enter your retina, and focused to a point that is REALLY DANGEROUS. The heat from the light hitting your retina will immediately denature all the proteins inside your retina cells, killing them.
This will happen no matter how much IR or UV you block.