You ever wonder why you can see your own blood vessels when you get an eye exam? No? Well I’m going to pretend you are wildly interested in this very specific topic.
This is what you see when you look at the back of the eye. Well, not you, but definitely me. This is what I see. A bunch of arteries and veins extending from the optic nerve across the entire retina. 
If we take a cross section of the retina, it looks exactly like this, with words in it and everything. Light comes in, traverses the entire thickness of the retina before hitting the photoreceptors. You’ll notice the blood vessels sitting on top
When you shine a pinpoint light source into the eye, the vessels get in the way and cast a shadow onto the deeper layers.
Normally your brain would ignore this shadow because it knows those are just the retinal vessels. No need to make the human see those. But when the light moves around (like we do during an eye exam) the shadow moves, tricking your idiot brain into processing the image.
And then you end up saying, “whoa I can see my blood vessels!” and then we say, “yeah pretty cool,” in an extremely half-hearted way.
