The secret to why we see the world in 3D is this: Light enters our eyes at angles.

Imagine you are looking straight ahead at a box.

Now imagine what would happen if light traveled towards your eyes in straight parallel lines.

In the picture above are rays of light drawn in red and green. The rays of light are reflecting off of the box and traveling in parallel lines towards your eye.

If light traveled in straight parallel lines, as illustrated above, you would only be able to see the bottom portion of the box, as indicated by the rays of light drawn in red. The rays of light drawn in green would pass right over your eyes. You would not be able to see the top portion of the box.

However, from experience you know that in real life you would be able to see the entire box. The only way this is possible is if light entered your eyes at angles, as illustrated below.

In this picture, the rays of light are drawn reflecting off of the box and towards your eye at angles. This allows rays of light reflecting off every single part of the box to enter your eye. In this way, your eye can see the entire box.

Imagine you placed a sheet of glass between yourself and the box.

If you were to take a magic marker and trace the box you see onto the glass, you would have drawn a 3D perspective drawing of the box onto the glass! The sheet of glass can be thought of as a painter's canvas. (In fact, many painters prefer glass as a canvas!) Each line traced onto the glass represents a place where light reflecting off of the box passed through the glass while traveling towards your eye.

In the picture below, I have identified the place where a ray of light reflecting off the top of the box would pass through the glass.

Look what happens as the box is moved further and further away from the glass canvas.

If you think about it, you will see that as the box is moved further and further from the glass canvas, its height would be painted smaller and smaller onto the canvas.

What does this have to do with the way we see the world in 3D? Everything. The amazing fact is, your eye has its own canvas! This canvas is called your retina. Light passes through our retinas in exactly the same way that light passed through the glass canvas in the pictures above. This is why things look smaller as they get farther away, and larger as they get closer to your eyes!

Our retinas are literally the canvases upon which the world is painted!