Gradient Bar

What is happening

The horizontal bar is a rectangle filled with a single color. Only the background shades from dark to light, left to right. And yet the bar looks like it has the opposite gradient — “brighter on the left, darker on the right.”

Cover the background with your finger and look at the bar alone: you can confirm it’s a single color instantly. This is easy to try right on your phone.

Why it looks that way

The same mechanism as the brightness contrast illusion is happening continuously, position by position. The left end of the bar touches a dark background, so it reads as relatively bright; the right end touches a light background, so it reads as relatively dark. That evaluation blends smoothly across the bar’s interior, producing a gradient that doesn’t actually exist.

Make one yourself

This illusion is determined entirely by the background’s two colors and the bar’s color and thickness. In any drawing app, lay down a gradient from dark on the left to light on the right, then place a single bar of mid-gray on top — the same effect appears. The thinner the bar, the more each part of it is pulled by the background right next to it.

First described

After Michel Eugène Chevreul, 1839
Source: "De la loi du contraste simultané des couleurs" (The Law of Simultaneous Contrast of Colors)

The starting point was Chevreul's work as dye-shop director at the Gobelins tapestry works, investigating complaints that thread colors looked different next to other colors.

See it in the timeline