What is the Muller-Lyer Illusion as noted in the nonfiction book, Thinking, Fast and Slow?

The Muller-Lyer Illusion is a famous representation about how the eye can trick the mind. The illusion consists of two straight lines drawn horizontally on a paper. The top line has short lines, or fins, attached to either end pointing inward; the bottom line has the same length short lines attached at the ends pointing in an outward direction. Most people pick the lower line as being longer than the top line. But it’s an optical illusion; the horizontal lines are exactly the same length. Even after measuring the lines and know them to be the same, System 1 thinking will still believe the lower line to be longer. System 2 thinking overrides System 1 thinking and can use this exercise in the future to understand the impact of illusory patterning.