Imagine a grandfather slowly losing his sight to cataracts. The thing he wants most is to see his first grandchild. Frustrated, he sits at home, unable to support himself, cared for by his son.

Stories like this are common in a world where 39 million people are blind and another 246 million have low vision that impairs their daily lives. In China alone, more than 8 million people are blind and more than 67 million people have low vision.

Almost half of those that are blind have cataracts - a natural clouding of the lens that usually comes with age. In high-income countries, fewer people go blind from cataracts than in low- and middle-income countries like China. That's because the people in high-income countries have greater access to the simple operation that replaces the clouded lens with a plastic one.