Two identical orange lines that your brain is absolutely convinced are different sizes. Railroad tracks are involved.
What: The Ponzo Illusion is a gorgeous example of your brain being too clever for its own good. It was cooked up by Mario Ponzo back in 1911 [1], and it’s all about how your visual system uses depth cues to judge size, even when it really shouldn’t.
Those two orange horizontal lines? Exactly the same length. Your brain is doing the thing that makes the top one seem bigger. It’s seeing those converging railroad tracks and going “ah yes, perspective! The top thing must be farther away, therefore it must be bigger!” Except… it’s not. It’s the same.
When you’re animating the tracks it breaks the illusion a little bit, as you see the lines getting closer growing, but these staying the same, which I found fun. Sometimes I feel like animation can massively enhance, but in this case, the opposite.
The wikipedia page [2] has a cool image of the lines over a real life track, and that honestly makes the effect a lot more clear, i guess because the more realism, the more my brain ebrances the perspective.
How To: Just look at the two orange lines and try to convince yourself they’re the same length. Good luck! You can toggle “Show Same Lines” in the controls to see the proof, vertical lines connecting their endpoints showing they’re identical.
Increasing the track seperation should help the illusion (it does for me), which I guess makes sense as it does increase the “perspective”.
Explain it: Your visual system has spent your whole life learning that things farther away look smaller. When you see converging lines (like train tracks disappearing into the distance), your brain automatically kicks into perspective mode.
The top line sits where “far away” things would be, and the bottom line sits where “close” things would be. Since they’re making the same size image on your retina, your brain does some quick math: “if the far thing looks the same size as the close thing, the far thing must actually be BIGGER!” And boom, illusion achieved.
The experts are calling it “size constancy” and it’s normally super useful, it’s why people don’t appear to shrink as they walk away from you, your brain does the math.
Fun fact: This same illusion might explain the Moon Illusion! When the Moon is near the horizon with trees and buildings around, it looks huge. Up in the empty sky? Smaller. Same Moon, same size, just different depth cues messing with your head.
I've researched these optical illusions in my spare time but am clearly not any kind of expert and my explainations are pretty smooth brained, if you find something mis-cited, earlier examples, or general mistakes please new let me know via toymaker@toms.toys, be kind!