Eluzions Illusions

Some two-dimensional figures can be interpreted as solid objects in more than one way. A well-known example of this is the flat representation of a wire-frame cube, which can be seen as if from above, or below.

The Necker Cube

is named after the Swiss crystallographer Louis Albert Necker, who in the mid-1800's saw cubic shapes spontaneously reverse in perspective. The human mind perceives the 2-dimensional shadow of a cube as a 3-dimensional object, but the 2-D image doesn't distinguish the front and back faces. You can perceive either face as being in front.

The Animated Necker Cube

This applet forces you to periodically switch your perception of the cube. It does this by moving images horizontally and vertically through the cube in a mutually inconsistent way.

Here is a rotating 3d Necker Cube.

You might see this as a wireframe cube rotating in 3d space, complete with perspective effects. However, you can, either by an effort of will, or chance, make the 'back' face (the smaller square) appear to become the 'front' face, and vice versa. If you succeed in this, or you started out seeing it that way, then the object appears to distort strangely as it rotates!

Cube in a Corner

This picture is ambiguous in three ways:

• As a small cube in front of a larger cube;
• As a large cube with a corner cubelet cut out;
• As a small cube inside three perpendicular adjoining planes.

Optical Illusions Poster