This optical illusion was discovered by the German physiologist Ewald Hering in 1861. The two blue lines are both straight and parallel to each other. If you are like most folks you may see things differently. The lines in fact may look as if they are bowed outward.
The distortion is produced by the lined pattern on the background, that simulates a perspective design, and creates a false impression of depth. If you pass your mouse over the image the background lines will be removed and you can see for yourself that the blues lines are definitely straight.
Project: Create your own illusion. Select a sheet of paper and place a dot in the center of the sheet. Now draw 10 lines through the center, mimicking as close as you can the background of the above illusion. Now on a sheet of clear document protector draw your two vertical lines.
Increase the amount of background lines to 18. Does that affect the illusion?
Does changing the color of the background change the effect?
Does changing the color of the background lines change the effect?
Does using two different colors for the vertical lines alter the illusion?
Try fat lines. Try skinny lines. Does the thickness of the lines affect this illusion?
Vary the distance between the vertical lines, does this change the effect?
Does the distance from which you view the illusion change the illusion?
What can you conclude, if anything, about how we perceive things from this project?