Today's Google Doodle celebrates the work of Ferdinand Monoyer. You will probably never have heard of him (until today) but this French ophthalmologist is very important in the world of optics.

He invented the dioptre (the scale of measurement of visual acuity) based on how far away from something you need to be to see it clearly.

He also invested the eye chart that helps measure visual acuity - and which has been used around the world by billions of people. So when you next have an eye test, you'll now know who invented the chart they ask you to look at!