Optical Illusions

Optical illusions are not mistakes of the eye; they show the clever shortcuts the visual system uses to make sense of the world quickly. This interactive gallery brings together classic illusions, each with a short explanation of what it tells scientists about how vision works.

You will find the Hermann grid, where grey dots seem to flicker at the crossings of white lines; the Müller-Lyer illusion, where lines of equal length look different because of their arrowheads; and the Ebbinghaus illusion, where a circle looks bigger or smaller depending on its neighbours. The café wall makes parallel lines appear tilted. Our own original motion illusion drawing, in the style of the well-known rotating snakes, seems to move although it is completely still. The afterimage demo shows what happens when photoreceptors adapt to a colour, while Troxler fading and motion-induced blindness show how unchanging objects at the edge of vision can simply disappear from awareness.

Many illusions let you change parameters, hide parts or time your observation, so you can test them yourself. They are a fun way to learn, and great for children and classrooms. They are not a diagnostic tool: experiencing an illusion more or less strongly than others is normal and says nothing about the health of your eyes.

How to use

1. Pick an illusion from the gallery and read the short instruction above it.
2. View it at a comfortable distance in a normally lit room, with both eyes open unless told otherwise.
3. Follow the specific task, for example staring at a point for 20-30 seconds for the afterimage or Troxler fading.
4. Use the controls to change the illusion and see when the effect gets stronger or disappears.
5. Read the explanation to learn which part of the visual system creates the effect.
6. Take a break if motion illusions cause discomfort, dizziness or headache.

Reading your result

There is no right or wrong result. Most people experience most of these illusions, though the strength varies between individuals and with age, attention and viewing distance. Seeing an illusion only weakly, or not at all, is not a sign of an eye problem. Comparing your experience with friends or classmates is part of the fun.

If you notice unusual visual effects in daily life that are not caused by an illusion, such as persistent afterimages, shimmering zigzag lines, flashes of light or objects that seem to move when they do not, mention them to a doctor. These can be linked to migraine aura, retinal problems or neurological conditions.

Frequently asked questions

Why does the still picture seem to move?

Certain repeating patterns of contrast are processed at slightly different speeds by the visual system, which the brain interprets as motion, especially in peripheral vision and during small eye movements.

Why do things vanish in Troxler fading?

Neurons adapt to unchanging stimuli. When you fixate steadily, blurry objects in your peripheral vision stop being signalled and the brain fills in the background instead.

Are optical illusions bad for the eyes?

No. They cannot damage your eyes, though some motion illusions can feel uncomfortable for sensitive people.