What you’re seeing begins with a simple rule of physics, light travels in straight lines. Outside, every point of the scene is sending light in all directions. When there is only a tiny opening, like a gap in curtains, only a narrow bundle of rays from each point can enter. These rays cross at the opening, so light from the top of the scene lands lower on the wall and light from the bottom lands higher, creating an inverted image. Because the opening is small, the rays do not scatter much, so the image stays sharp but dim. Inside your eye, the same geometry is happening but with biological precision. Light enters through the pupil and is bent by the cornea and lens. This bending, called refraction, focuses the rays so that each point in the outside world maps to a precise point on the retina, again forming an upside down image. The retina then converts this light into electrical signals using photoreceptor cells called rods and cones. These signals travel through the optic nerve to t...