Impressionist paintings look blurry up close because the artists laid down short, separate strokes of pure color instead of blending them on the palette. Standing close, the eye sees the individual strokes. Stepping back, the strokes merge into a single, more accurate impression of light.
The trick relies on how human eyes mix color, not on how the paint sits on the canvas.
The science behind it
In 1839, the chemist Michel Eugène Chevreul published a study on how colors placed side by side change how the eye reads them, a principle called simultaneous contrast.
Impressionist painters leaned on this. Instead of mixing orange on the palette, they placed strokes of red and yellow next to each other and let the viewer's eye do the mixing.
I find it strange that a color theory written for chemists ended up deciding how a room full of paintings looks from ten feet back.
Speed mattered too
Painting outdoors, in front of a changing scene, left little time for careful blending. The light a painter was chasing could shift within minutes.
Quick, separate strokes were also just faster than smooth, layered ones.
No outlines, on purpose
Academic painting built forms with contour lines. Impressionist painters mostly dropped the line and built form out of color changes instead, which is part of why the edges look soft rather than blurred.
Where the idea went next
A decade later, painters like Georges Seurat pushed the same idea further, breaking color down into small dots instead of strokes, a technique now called pointillism.
See the paintings that made this technique famous in Famous Impressionist Paintings.