Or… is it a system of valleys and canyonlands cut into the surrounding plains?

And here's another – is this text/image cut into the wood, or is it bumping out?

Like the famous duck/rabbit illusion, it can sometimes be hard to perceive both sides of

these illusions, so let's try one more: a series of plateaus on earth?

Or the grand canyon?

These are examples of a multistable perceptual illusion that often comes up when looking

at shaded relief maps or aerial and satellite photos of terrain.

So why do we sometimes see these features as popping up or out of the image, and why

do we sometimes see them as cut or dented in?

There are essentially two reasons: First, the fact that on earth the sun is always overhead,

and second, a weird symmetry of light and shadows.

Ok, so if you're a human who grew up on earth, you're used to the fact that light

mostly comes from above.

I mean, the sun is always overhead, illuminating things from above, or above and to the side,

but never from below the horizon, which means that shadows are pretty much always on the

bottoms of things.

This is so ingrained in us and our culture that religion talks about falling down into

darkness and being raised up into the light, western art exhibits a “top-left lighting”

convention, as does computer interface design, and the international space station has lights

on one side to help give floating astronauts a consistent sense of “up” and “down”.

And “Shadows on the bottom, light on the top” gives our brains a way to perceive

the 3D-ness of features just from the position of their shadows.

If there's a bump sticking out from a wall, the shadow will be on the bottom of the bump.

If there's a dent in the wall, the shadow will be on the bottom of the overhang, which

means the TOP of the dent.

Roughly speaking, when we have no other context and see shadows on the bottom of a feature,

we perceive it as convex, coming towards us, and when we see shadows on the top of a feature,

we perceive it as concave, or dented away.

But there's a cruel symmetry in nature: a concave feature, lit from one side, can

cast very similar shadows to its convex counterpart, lit from the other side . A bump lit from

above has shadows on the bottom, and a dent lit from below also has shadows on the bottom,

which makes it easy to misperceive a dent lit from below as a bump, or a bump lit from

below as a dent.

Similarly, a mountain range lit from the east has shadows on its western slopes, while a

valley lit from the west also has shadows on its western slopes.

Not really a problem when you're standing next to the mountain or valley.

But viewed from high enough above, all our brains really have to go on for the concavity

or convexity of landscape features is their shadows.

So we'll tend to see these geographic features as mountains when the shadows are on the bottom,

and valleys when shadows are on the top.

Which, of course, is only correct if the light happens to be coming from the top of the image.

In fact, the mountain or valley illusion is so strong that shaded relief maps of the northern

hemisphere generally show light coming from the north, a direction the sun never actually

shines in most of those places.

Cartographers are more than willing to sacrifice the accuracy of the sun's position to ensure

that they accurately communicate the geographic features they're mapping.

And it's good they do – I actually decided to make this video after I was looking at

a map upside down from across a table, and my brain thought that all the valleys were

mountains and mountains were valleys!

I had to get up, clear my head and go around to the other side of the table before I could

get them to switch to being the right way round.

So if you're confused why a map or aerial or satellite photo looks weird, try rotating

it 180° to see if it makes more sense!

Hey, Henry here, today I thought I'd try something different.

I've been wanting to learn how to use Adobe character animator for ages, and it just so

happens that this video is sponsored by the online learning site skillshare and they have

lessons on character animator.

No joke, as of this morning I had no idea how to use this software, and now you're

seeing me, in stick figure form, animated in character animator.

So yeah, skillshare clearly has lessons for learning stuff, and not just animation: cooking,

photography, programming, etc.

It's about 10 dollars a month, but you can get two free months by going to skl.sh/MinutePhysics,

which also lets skillshare know you were sent there by me in animated stick figure form.

Bye!