Optical Illusions: the site explaining 156 visual tricks

Optical Illusions & Visual Phenomena is a single, carefully maintained archive of 156 visual tricks, each one demonstrated in the browser and explained by the retired vision scientist who built it. It is worth a look because it does the thing most illusion pages refuse to do: it tells you why the illusion works, in plain language, and then gets out of the way. There is no quiz, no leaderboard, and no attempt to keep anyone scrolling. Just the trick and the reason for it.

In brief

• The site holds 156 optical illusions and visual phenomena, each with a written explanation by Michael Bach, a retired professor of vision science.
• Illusions are sorted into eight categories, from motion and colour to cognitive effects and a short set of auditory tricks.
• It has been online since 1997, is built in plain JavaScript and HTML5, and is, in its own words, intentionally free of adverts and tracking.
• It treats each illusion as a small experiment in perception rather than a party trick, which is what keeps people reading long after the novelty fades.

What the site actually is

The front page of the collection states its own scope without ceremony: 156 optical illusions and visual phenomena, with explanations by Michael Bach. That header is the whole promise. There is no splash screen, no sign-up, no cookie wall. A visitor lands on a plain index and starts clicking.

Each entry is a self-contained page. The illusion runs live, usually with a slider or a checkbox to adjust it, and the text underneath explains what the eye is doing. The tone is that of a patient lecturer, not a showman. Nothing shouts. The restraint is the point, and it is rare enough on the modern web to feel almost radical.

The archive does not try to be comprehensive in the academic sense. It is a personal selection, assembled over decades, of the phenomena one vision scientist found worth demonstrating. That editorial hand is what gives it coherence.

There is also a quiet confidence in how little the site explains about itself. It does not pad its pages with mission statements or testimonials. The work is the argument. A visitor who has never heard of its author can still tell, within a minute, that the pages were made by someone who cares about getting the details right.

How 156 illusions are organized

The collection sorts its entries into eight categories, and the counts give a fair picture of where perception is most easily fooled. Motion and time leads with 36 illusions. Luminance, brightness and contrast follows with 27, and colour with 26. The spatial and geometric families are smaller but dense.

The eight families

• Motion & Time, the largest group, covering apparent movement, aftereffects, and timing errors.
• Luminance, Brightness & Contrast, where identical greys refuse to look identical.
• Colour, including afterimages and the way surrounding hues bend a patch of colour.
• Geometric & Angle Illusions, the classic lines and angles that measure wrong.
• Space, 3D & Size Constancy, where depth cues quietly rewrite size.
• Cognitive and Gestalt effects, where the brain fills in what is not there.
• Specialties with faces, a small set exploiting how strongly people read faces.
• Auditory Illusions, a short reminder that ears can be fooled as easily as eyes.

The grouping is practical rather than theoretical. A reader can wander one family at a time and start to see the mechanism they share, which is more instructive than a flat alphabetical list would ever be.

The lopsided counts tell a small story of their own. Motion, luminance, and colour dominate because those are the channels the visual system processes earliest and most aggressively, which is exactly where it cuts the most corners. The smaller families, faces and auditory tricks among them, are no less striking, but they show the brain being fooled at a higher, more interpretive level. Reading the categories in that light turns a menu into a rough map of how sight is assembled, stage by stage.

The illusions people remember

Many of the entries are famous, and seeing the canonical versions in one place is part of the appeal. The Rotating Snakes illusion, in which static rings of colour appear to drift, sits alongside the Motion Aftereffect, the waterfall effect that makes a still scene seem to flow upward after staring at motion.

The geometric standards are all present. The Mueller-Lyer illusion, where two equal lines look unequal because of their arrowheads, and the Necker Cube, which flips between two orientations as you watch, are both there to be prodded. So are Hermann's Grid, with its phantom grey dots, and Kanizsa's Triangle, a shape the brain insists on seeing where no edges exist.

There are quieter stars too. Hinton's Lilac Chaser, in which a moving gap conjures a green dot that is not there, and the Shepard Tone, an auditory illusion of a pitch that forever rises without climbing. The collection even includes The Dress, the photograph that split the internet in 2015 over whether it was blue and black or white and gold.

Seeing them gathered in one place changes how they read. A single viral illusion feels like a stunt. A hundred and fifty-six of them, sorted and explained, start to look like evidence. The pattern that emerges is that vision is not a camera faithfully recording the world. It is an interpretation, constantly guessing, and the guesses are usually so good that the failures are the only time anyone notices the machinery at all.

The one that went viral in its own right

Not every entry is a textbook classic. In January 2023 the site's Pigeon Neck illusion reached the front page of Hacker News, where it drew 344 points and a long thread of people testing it on themselves, according to the discussion there. The effect, dated 2014 on the site, shows how a repeating pattern can make a bird's neck appear to swivel unnaturally.

It is a useful illustration of how the archive works in the wild. A single page, no heavier than it needs to be, can circulate for years and still surprise a fresh audience. The illusion does the convincing; the explanation underneath does the rest.

Why explanations matter more than the trick

Most illusion pages stop at the reveal. This one begins there. Under each demonstration, Bach describes the perceptual machinery involved, often with a note on competing theories and what remains unsettled. The writing assumes curiosity but not a degree, which is a difficult balance to hold across 156 entries.

That editorial discipline turns the site from a toy box into a reference. A student can cite it, a designer can learn why a layout feels unstable, and a casual visitor can simply enjoy being fooled and then understand the fooling. The explanations also model honest uncertainty: where science does not yet know the full story, the text says so rather than inventing one.

The result is a rare thing online, a page that respects both the wonder and the reader. It never treats the illusion as a cheap gotcha.

What the illusions reveal about everyday sight

The practical value of the archive is that its tricks are not exotic. The same shortcuts that make Hermann's Grid flicker are at work every time a reader glances at a spreadsheet or a train timetable. The brain's habit of sharpening edges, inventing contours, and smoothing motion runs constantly, below awareness, and the illusions simply catch it in the act.

That is why designers, photographers, and anyone who arranges information for a living tend to linger here. Understanding size constancy explains why a layout can feel cramped when the measurements say it is not. Understanding simultaneous contrast explains why a grey that looks right on one background looks wrong on another. The illusions are edge cases, but the lessons are ordinary.

None of this is pitched as a productivity tip. The site never reaches for a takeaway. It trusts the reader to carry the insight out on their own, which is a more durable kind of teaching.

Who Michael Bach is

The author is not an anonymous hobbyist. According to his personal site, Michael Bach is a retired professor of vision science, long affiliated with the Eye Center at the University of Freiburg's Medical Center in Germany. His research career in visual perception spans decades of published work.

He is also the author of FrACT, the Freiburg Visual Acuity and Contrast Test, a free tool used in clinics and labs to measure how well an eye resolves detail and contrast. The illusions collection sits in the same tradition: rigorous about vision, generous with access, and unbothered by commercial gloss.

That background is why the explanations carry weight. They are not scraped summaries. They come from someone who spent a career measuring exactly how human sight succeeds and fails.

How the site is built, and why it stays fast

The collection has aged gracefully because it was built to. It has been online since 1997, with the first installment dated that April, which makes it one of the longer-running science sites on the public web. In 2018 the author retired the last of the Flash versions, and in 2019 he rebuilt the whole thing in Jekyll, the static-site generator.

The illusions now run on plain JavaScript and HTML5, which means they work in a current browser with nothing to install and no plugin to chase. Pages are light. They load quickly, behave predictably, and do not pester the visitor. The site also states plainly that it is free of adverts and tracking, a choice that keeps it quiet in every sense.

This is quietly instructive for anyone building for the long term. Static files, open standards, and a refusal to bolt on dependencies are why a page from the late 1990s still opens cleanly today.

The Flash migration is the telling detail. A great deal of interactive web work from that era simply died when browsers dropped the plugin, taking a generation of educational demos with it. Rebuilding every illusion in native web technology was unglamorous labour with no reward beyond survival. The archive did it anyway, which is why it is still here while so many flashier contemporaries are now broken links.

How it reached a wider audience

The archive has surfaced on Hacker News more than once. In November 2020 the collection's index was posted there and drew a round of appreciative comments, as recorded in that thread. The reception followed a pattern familiar to anyone who has watched a genuinely useful site circulate: less hype, more people quietly bookmarking it.

What spreads is rarely the newest entry. It is the combination of a striking demonstration and an explanation a reader can trust. That pairing travels well, which is why the site keeps reappearing in link roundups and classroom reading lists years after any given post.

The longevity is partly a function of credibility. Illusions circulate constantly on social platforms, usually stripped of any source and often with a wrong explanation attached. A visitor who traces one back to this archive finds the opposite: a named author, a research pedigree, and text that is careful about what is known. In an environment thick with confident nonsense, that reliability is its own kind of draw.

How to explore it

The best approach is to pick one category and work through it slowly. Motion and time rewards patience, because many of its effects need a few seconds of steady gaze before they bloom. Colour and contrast are better in a dimmed room, away from glare.

Readers should treat the sliders and checkboxes as the main event, not decoration. Turning an effect off and on, or pushing it past its comfortable range, is where the explanation underneath starts to make sense. The illusions are designed to be manipulated, not merely watched.

There is no correct order and no finish line. The archive is built for wandering, and the 156 entries are enough to reward return visits for a long time.

It also makes a fine thing to share with a child or a sceptical friend. The demonstrations are instant and physical: a line that will not stay straight, a colour that drains the moment you look away. Nobody needs the vocabulary to feel the effect. The explanation is waiting for whoever wants it, and ignorable for whoever does not.

Why it belongs in the boring-website canon

Optical Illusions does one thing with complete seriousness and no noise, which is the quality this blog keeps looking for. It is single-minded, lovingly maintained, and quietly brilliant, the same spirit that runs through the wider Boring Websites network. A site can be modest in its ambitions and still be the best in the world at what it does.

It also makes a gentle argument about the web itself. A page does not need to be loud, tracked, or monetised to be worth returning to for 29 years. It needs to be useful, honest, and fast. By that measure the collection is a small masterclass.

FAQ

How many illusions are on the site?

There are 156 optical illusions and visual phenomena, each with its own page and a written explanation. The site states the count in its own header, and the entries are divided across eight categories, the largest being motion and time. New ones are added occasionally, so the total has grown slowly over the years.

Who made it, and is it trustworthy?

It was made by Michael Bach, a retired professor of vision science formerly at the University of Freiburg's Eye Center in Germany, and the author of the FrACT visual acuity test. The explanations reflect a research career in perception, and the text is candid about where the science is still unsettled.

Does it cost anything or show ads?

No. The site is free to use and states that it is intentionally free of adverts and tracking. It has been running on donated time since 1997.

Will the illusions work on my phone?

Generally yes. The illusions were rebuilt in plain JavaScript and HTML5 after Flash was dropped in 2018, so they run in any current browser without a plugin. A larger screen and a dim room make some of the subtler motion and contrast effects easier to see, and a few are best viewed with steady attention rather than a quick glance.