The Cornea and Lens
This video presents the same text shown beside it, spoken and on screen. It adds nothing the text does not say.
State
The cornea and lens focus incoming light onto the retina, with the lens changing shape to focus at different distances.
Show
The lens thickens to focus on a page held close and flattens to focus across a room.
Watch for
Focusing is not detection. A perfectly focused image on a damaged retina produces nothing.