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Mystery of the Senses—Vision

Classroom Activity

Objective
To view six optical illusions and explain why the visual system might interpret them the way it does.

• copy of "Optical Illusions" student handout ( HTML)

1. Copy and distribute the "Optical Illusions" student handout to each student.

2. After students have tried each illusion, explain that the first three illusions distort our sense of perspective, because they deceive us into seeing what we expect to see. The last three illusions provoke our retinas to respond to light and movement.

Students should make the following conclusions about each illusion.

• Faces and vase: Our brain identifies an object by separating it from its background. Depending on which portion of the picture you choose to "see" as the background, the picture will appear as one or the other image.

• Two lines: The two lines are actually the same length. However, we are trained to interpret two-dimensional drawings as three-dimensional images. Therefore, our brains perceive the line on the left as shorter because it appears closer. When the distance of an object is halved, we perceive its size as doubled.

• Lines with hash marks: The opposing directions of the hash marks confuse the brain cells that control orientation, so the lines don't look parallel.

• Shadow: Only the shadow exists in this illusion. We assume that the letters exist too, so our brain perceives them in the illusion.

• Grid of black squares: Actually, there are no spots, but the contrast between black and white causes our eyes' retinal cells to create the illusion of spots.

• Spiral: When we stop rotating the spiral, the receptors in our retinas that signal movement to the brain reverse the movement we saw when we rotated it.