More than Meets the Eye

An optical illusion is the shift we experience between what our eyes see and how our brain processes that information. As we perceive the world around us, our brain receives 3 dimensional clues about our surroundings, including depth, shading, lighting and position, which enable us to see objects correctly. When viewing a 2 dimensional image, however, we can be tricked into seeing things differently or even into seeing things that aren't even there. While there is a wide variety of illusions, here are a few classic examples:

 

Multi-stable Optical Illusions: In a Multi-stable Illusion, your brain perceives an illustration in two different ways. In this example, Corner House by István Orosz, it appears that the walls either come together in a way that the windows face inward, toward each other, or, as two walls facing outward away from one another. With crafty use of perspective, the image tricks your eyes into shifting between the two options. If you cover up the bottom half of the image, you can see it would be impossible for the windows to be facing away from each other and conversely, if you cover the top half of the image, it would be impossible for the windows to be facing each other. This combination is what tricks your eyes into seeing it flip flop between the two. Click here to learn more about him and see other examples of his impossible art. 

Hermann Grid Illusion: This Illusion was created by Ludimar Hermann in 1870. In this illusion, your eyes are tricked into seeing grey dots appear within the white intersections of the grid. If you were to now stare directly at one specific intersection, the grey dot within that intersection disappears altogether.  For a detailed explanation as to how this illusion works, click here.

AfterImages: An afterimage occurs when your eyes are focused on a stimuli and when that stimuli is removed, you see either a reverse or contrasting image to what was there previously. In this example, if you stare at the flag image shown for 30 to 60 seconds, then look towards a white surface, you will see a the image of the flag in contrasting colors. For more examples of afterimages and how they work, click here.

Ebbinghaus Illusion: What you see here is one example of an Ebbinhaus Illusion. In this illusion, one plays with relative size to trick you into seeing something bigger or smaller than it actually is. This classic example of Titchener circles takes two orange circles and surrounds them with different sized gray ones. On the left, the orange circle appears much smaller and on the right, the orange circle seems much larger, when in fact, they are the same exact size. These illusions work similarly to Ponzo illusions, which play on the use of perspective to accomplish the same task. Click here for a nice video that demonstrates the Ebbinghaus Illusion in action. 

Motion Illusion: A motion illusion is one in which an image appears to in motion due to the way our brain processes interacting color contrasts, shape and position. Illusory motion can occur in multiple ways depending on the image. Some can appear to turn, as in the example provided, while other can appear to flicker, pulsate or shift. Click here for some other great examples of Motion illusions.

Ambiguous Illusions: Ambiguous Illusions demonstrate how our brain can process images in completely different ways.  Look at the example by Oleg Shuplyak. At first glance you may see all the fine details of a little bird sitting on a branch near colorful leaves. Now, look at the image again, and look at the leaves again. You may notice now, that they also take on the appearance of a second bird. Some might see two birds at first before seeing that the illusion of a bird is created with an illustration of leaves. If you’d like to see some more of ambiguous artworks, click here.

Paradox Illusions: Paradox illusions are such that create an impossible shape. One classic example of this illusion is the Pemrose Triangle you see here. These impossible shapes could never exist in the real world, only as a 2 dimensional image. At first glance, it flows wonderfully, but upon closer examination, you notice that the the way in which the object is created, could never truly exist. Click here to view some other great examples.

Optical illusions have interested people for many years. Here are a few great resources to see the wide variety available and learn more in depth how they work:
Wikipedia, Illusion Index, How Stuff Works