The mountain or valley illusion is all about what light position does to shadows.
Our brains are wired to believe that light generally comes from above. This makes sense here on Earth, light from the sun pretty much always is coming from either directly above us, or above us at an angle.
This is such a persistent phenomenon that we use it to determine the shape of objects. If an object has a shadow beneath it, we assume it is convex, whereas if it has a shadow above it, we assume it is concave.
This, as explained in the MinutePhysics video above, is why we struggle to discern whether a formation is a mountain or valley when a photo is taken from far away, like space.
Cartographers who make relief maps even orient their drawing’s light in a place it would never naturally occur, just so we can understand what’s sticking up and what’s hollowed out.
It’s pretty nuts to consider how much of our perception is based on our very specific experiences living on this particular rock in space, and how different our experience would be otherwise.
Video credit: MinutePhysics
This article was originally written for and published by Popular Mechanics USA.