Can You Solve This Shadow Illusion?

We have that card with a little round hole in it, and what I want you to do is hold it up, um, and try to cast a shadow on that wall there. I want you, before you do it, to predict what we're going to see. Shadow of the card? Shadow of the card with a little hole in it? We will see that figure enlarged on the wall. It's not very pronounced, but you can actually do see the little hole in it—and of course me!

So, Cole, woohoo! My next question for you is: we have this card. What's that like? It's big, a hole! Can you tell me what size circle we're going to see, like, compared to the other one? Bigger—like twice the size maybe, but still the card with a larger round hole. What do you see? Bigger hole: the card and the hole slightly bigger than the previous one—the same thing, just larger.

Okay, what I want you to do is the same size though; it's approximately the same size as the last one. Why don't we compare it? Here you go. You're right, it is—the one's just fainter than the other. Oh goodness, it actually looks about the same—same size hole! I can't believe that—it's must be the sun playing tricks with my mind.

What about here is really going to test you, right? Okay, uh, tell me about what's on that card. It's a—before you put it up—a triangle? Yeah, okay, so what do you expect we will see on the wall? A triangle, but I wouldn't be surprised if it was a circle. What are we going to see? A triangle, I'm certain.

Okay, let's have a look. Oh wow, that's awkward for me! It's a circle! It'll be a triangle. Well, we're not going to see a triangle. Why not? 'Cause everything else has been a circle, so it'll be a circle. I'm sticking to a triangle! Okay, it's a circle. Look, I'm interested. Well, there you go, it's a circle. Do you want to compare it to the one of the others? I'll give you the bigger one.

Oh my goodness, I really like this project! I'm learning something. The sun is definitely playing tricks on my mind. The sun is deceptive. That's weird. Why are we seeing circles with all these cards, regardless of the shape of the hole? I really don't know; I'd like to know. When you take away the definition from any kind of shape, it ends up being just round to our eyes.

There, for example, you get a perfect triangle, but it was a pretty decent circle from back. So, it must—you must get to a certain size. Here, it doesn't matter what shape it is, it's just turned back probably. Why is it going into a circle, though? Look at the sun; it looks around, doesn't it? So, I'm not sure if that's got anything to do with it.

Well, the light is going through these little holes, and it is the sun. The sun rays just sort of, um, going through the little holes making a circle of the sun. I don't know; you told me exactly. You nailed it! What we're looking at there is actually an image of the sun.

Really? Yeah, oh wow! The Earth's round and the sun's round, and yeah, the sun—the sun's round, I guess. The building's round? The building's round. Is it? No, the building's not round! So, what we're seeing is a projection of the sun. I've never thought about it before.

I think that you actually know, and I think you have actually thought about it before, but I think you're not queuing that right part of your brain that knows the answer. Yeah, probably! There's a part of your brain that knows the answer.

Well, I did photography at school, um, and we did pinhole cameras. The hole actually reflects what it's showing on the wall. So what's that? So it's the sun! Well, there you go, I learned something today, Derek.