How come when I close one eye and put my finger really close to the other eye I can see a translucent outline of my finger?
I can understand with two eyes how there are some parts that are only seen by one eye, but I don’t understand what is happening with a single eye open.
Lack of focus causes blur.
Let’s say that you’re looking at a small, distant object. Light from that object can hit any spot on the lens of your eye, and it’ll still be bent to hit the same spot on your retina. That’s what a lens does.
But now put your finger right in front of your eye. Some paths from the distant object to parts of your lens miss your finger, but some other paths are blocked by your finger. So that spot on your retina is now getting light both from the distant object and your finger.