The Poggendorff illusion is a visual phenomenon where a diagonal line appears misaligned when it intersects with two vertical lines. This illusion occurs even though the diagonal line is continuous and straight. The brain interprets the angles and positions of the lines in a way that creates a misleading perception of alignment. This illusion was first described by the German physicist Julius Poggendorff in 1860. It highlights how our visual system can be tricked by context, leading to errors in judgment about the position of lines. The Poggendorff illusion is often used in studies of perception and cognitive psychology to understand how we process visual information.