One of them, named the Helmholtz illusion, demonstrates that a square filled with horizontal lines will appear taller than a similar square filled with vertical lines (which is contrary to modern fashion advice). When squares contain both horizontal and vertical lines, the eye jumps from drawn line to imaginary line and the resulting figure seems to move.
For maximum effect, there needs to be sharp contrast between the two colors, which is why op-art artists typically pair black and white. These socks, named for Herrmann von Helmholtz, take a tamer approach, but feel free to choose whatever color combination suits your wardrobe. The sense of movement will remain.
Sizes: About 8¼” (21 cm) foot circumference, unstretched, and 9¾ (10)” (25 25.5 cm) foot length from back of heel to tip of toe (length is adjustable). To fit U.S. women’s shoe sizes 7½–8½ (9–10); European shoe sizes 39–40 (41–42). Socks shown measure 9¾” (25 cm).