Questions people actually ask

Britannica: the size of the image on the retina is one of the clues to distance; the larger the image, the closer we judge the object to be. Britannica: Space perception Delboeuf and Ebbinghaus are “size contrast” illusions; illusions that rely on depth and perspective belong to a different family. Michael Bach: Delboeuf illusion

The perspective explanation (Richard Gregory, 1968): fins pointing in look like the near corner of an object, fins pointing out like the far corner of a room. Michael Bach: Müller-Lyer illusion Britannica explains the Müller-Lyer illusion with the Gestalt ideas of convergence and divergence: the fins at the ends seem to lead the eye inwards or outwards, giving a false impression of length. Britannica: Müller-Lyer illusion

Asked to find the middle of the line in the Brentano version, big errors are completely normal: Michael Bach, whose illusion collection this comes from, says he is out by about 10%. Michael Bach: Müller-Lyer illusion It is one of the few illusions with a big age effect: for children under 10 it is weaker. Michael Bach: Ebbinghaus illusion

The moon illusion has two parts: the moon looks bigger than it really is, and much bigger near the horizon than high in the sky. Michael Bach: Moon illusion The moon’s image on a photograph is the same size whether it is low or high: the change happens in your brain. Exploratorium: Looking at the moon Birds, clouds and planes are closer when overhead and further away on the horizon, so we expect things on the horizon to look small. The moon doesn’t shrink there, so it seems huge. Exploratorium: Looking at the moon

Test it: at arm’s length, about two-thirds of the width of your little finger covers the moon, wherever it is in the sky. Michael Bach: Moon illusion If anything, the low moon is a tiny bit squashed from top to bottom, because its light passes through more air near the horizon. Michael Bach: Moon illusion

An Ames room looks like an ordinary square room from one peephole, but one side’s ceiling is about twice as high as the other. Exploratorium: Distorted Room Because the brain decides the room is square, people walking across it seem to grow and shrink. Exploratorium: Distorted Room

Adelbert Ames Jr was born in 1880 and worked on visual sensation and perception; he is best known for the Ames room. Exploratorium: Ames Chair (Science Snack)

Assuming size constancy, the visual system works out how big something is as its size on the retina multiplied by how far away it assumes it is. Michael Bach: Shepard’s “Subterranean Terror” Size constancy is always at work. It is why someone walking away from you doesn’t seem to shrink into a dwarf. Michael Bach: Size constancy

In the T illusion (also called the vertical-horizontal illusion), the upright stroke of an upside-down T looks longer than the crossbar, even when they match. Michael Bach: T-illusion Size constancy isn’t as good for up-and-down distances: from the top of a tower, people below look tiny, like toys. Michael Bach: T-illusion

It is named after Hermann Ebbinghaus, a pioneer of memory research, who probably found it in the 1890s but never published it. Michael Bach: Ebbinghaus illusion Edward Titchener printed it in a 1901 textbook, without claiming it as his, so it is also called the Titchener illusion. Michael Bach: Ebbinghaus illusion

Close one eye and hold a small coin and a large coin at different distances: you can make them look exactly the same size. Exploratorium: Size and Distance (Science Snack) The Exploratorium’s test: trace a rising full moon onto clear plastic held at arm’s length, then check again hours later. The high moon still fits the outline. Exploratorium: Looking at the moon