when light comes from a source and hits something it reflects the light while some of it doesn't. when the reflected light or a light which hasn't been reflected comes to a certin place in the eye called the pupil. there we accualy see it upside-down untill our brain turns it back to right-side-up. And then our brain tells us what the objects are. That is how we see.
Two bous were palying hide and seek:
1:"I see you!"
2:"Than where am i?"
1:"behind the tree"
2:"How do you know?"
1:"The light reflected from you and the reflected light hit my pupil. when the light came to my eyes it was sent to my brain and understood."