Notes

An optical illusion is when length, size, brightness, colour or motion looks different from what it really is. Short reads on the principles behind the works.

• What Is an Optical Illusion? Types and Why They Happen

  An optical illusion is not a visual bug but a glimpse of how we guess the world from limited information. Meaning, difference from illusion, 7 types and causes.

• Everyday Optical Illusion Examples — Where Your Eyes Fool You Daily

  A moon that looks huge, clothes that change color, a downhill road that looks uphill. Everyday illusions, paired with figures you can try here.

• The Moon Illusion — Why the Moon Looks Bigger Near the Horizon

  A newly risen moon looks huge, but not because it is closer or because of an air lens. How to check the illusion, with the leading theories and Ponzo figures.

• What Is Trick Art? Optical Illusions vs Trompe-l’œil and 4 Types

  Trick art is a term for pictures made to fool the eye. Difference from illusions and trompe-l’œil and four types: hidden, figure-ground, 3D, impossible figures.

• Spatial Frequency — Why Some Images Appear Only From Far Away

  Why fine patterns vanish and another picture emerges when you step back, explained via contrast sensitivity and low-pass filtering. The basics of hybrid images.

• Same Length, Different Look — Size and Context

  The size of a line or circle isn’t judged apart from the shapes around it. Four illusions compare length, area and width to show context’s effect.

• 3D Illusions — How Flat Pictures Seem to Pop Out

  Paper and screens are flat, so why do pictures seem to pop out or sink in? Perspective, shadow and overlap — the cues your brain uses to read depth.

• Same RGB, Different Color — Contrast, Assimilation and Shadow

  The same pixel value is not the same appearance. Compare contrast, stripe assimilation and shadow interpretation to see why no single explanation is enough.

• Straight Lines That Look Bent — How Background Direction Warps Shape

  You can make parallel or horizontal lines feel curved without bending them. Switch radial lines, diagonals and tiles and compare coordinates with perception.

• One Picture, Two Answers — Depth Reversal and Figure-Ground

  In an ambiguous figure, the same image can be read in more than one way. Use the Necker cube and the vase-and-faces picture to explore depth and figure-ground.

• Seeing Contours That Are Not Drawn — The Kanizsa Triangle

  A mere arrangement of shapes can make you sense an edge where no line exists. Compare the white triangle with figure-ground to spot the difference.

• Illusions That Change With Gaze and Time — Grids, Fading, Aftereffects

  Where you look and what you just watched can change how a picture appears. Explore grid spots, fading dots and the motion aftereffect under controlled viewing.