The photo above contains an optical illusion that isn't really visible to the regular eye, but to a graphic designer, the illusion serves an extremely important purpose.
Maybe the photo above will give you a better look. In the photo you can see that the bottom of the "C" hangs below the baseline and the top of both "C" and "A" extend just above the "L", or the cap-height. When a letter extends past the cap-height or baseline it is called "overshoot" or "overhang".
Although this is not your typical optical illusion, it is extremely important to every designer because it makes words appear optically pleasing. Without overhang from certain letters, some words would look very strange and become difficult to read in a fluid motion.
Franz Müller-Lyer, a German psychiatrist, performed an experiment in the 1880s, in which he asked people whether they thought the three lines below were all of the same length or whether one was longer than the others. He realized that by adding two chevrons that pointed inward at the end of a line segment made the line appear longer than a line with two or one outward pointing chevrons.
This became known as the Müller-Lyer Illusion. Letters form an overall shape, for example, an "H" forms a perfect square, but an "O" forms a square in the counter which appears smaller than the square that is created by the "H", which is why the "O" needs to have overhang to make the shapes optically appear the same size.
Here Kevin Larson shows the Müller-Lyer Illusion in action at the Seattle Nerd Nite Presentation last year.
Read more of Robbie Gonzalez's article: http://io9.com/the-famous-optical-illusion-hidden-in-every-typeface-in-1714788793
Watch Kevin Larson's entire Nerd Nite talk here: https://vimeo.com/79720653