Geometry

• riyan34:Flickr.com
  
  Geometric eye illusions involve shifting the eye's point of focus. This shift can cause straight lines to look curved and can cause objects to appear to be different lengths when they are actually the same measurement. For example, when a line is placed perpendicular to a line with the same length and width, the vertical line appears to be longer than the horizontal line because of the eye's natural inclination to associate objects running up and down with length.
  
  

Angles

• JustDreamingAway: Flickr.com
  
  Angular illusions often combine with geometric illusions to create a seemingly new image for the eye to interpret. Angles play a part in the appearance of an optical illusion by distorting the eye's naturally horizontal path so that any vertical line appears to lean one way or the other. This illusion is most obvious when looking directly at the picture. If the eye sees the picture from a distinct angle, the illusion is destroyed.
  
  

Color

• Optical illusions that appear using color are the result of contrasted or similar colors that alter the weaker color's appearance. These illusions are especially present when colored light is used. A room saturated in a red light will not appear to be as deeply red as the light actually is because it does not have another color present to contrast the existing shade of red. However, upon leaving the red-lit room, a plain room will appear tinted in green due to the eye's negative retinal afterimage, which is similar to viewing a photo negative.
  
  

Distance

• studionumber9: Flickr.com
  
  When a combination of illusions creates an image that makes an object appear closer or farther away than it really is, an optical illusion of distance is taking place. An illusion of distance also enables flat objects to appear three dimensional. The classic version of this illusion is drawing a horizontal line across a piece of paper and then drawing a triangle so that one side is the bottom of the paper and the other two sides are lines drawn from the bottom corners up to meet on the perpendicular line. This looks as if the triangle is a path that keeps going past the viewer's eyesight.
  
  

Natural Illusions

• Many optical illusions occur in nature that you witness every day. Trees generally appear taller while they are upright, but when they are cut down, they appear shorter. The stars in the sky appear to move slightly at night due to the stark contrast from the brightness of the stars to the darkness of the sky. Mirages are another form of natural illusion in which a heated surface expands the air around it and causes the light to distort its bend, creating a new image.