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Do you see a face or a vase? Photo: Bryan Derksen.
optical illusion:
Something that appears to a person to be different from what is really there. There are many different kinds optical illusions. Some are common daily occurrences, such as the way the moon appears to move in the sky as clouds float by in front of it. Other kinds of illusions are used by artists. Perspective is the way in which lines or shading in a picture are used to suggest depth or distance in a flat drawing. Optical illusions may also involve color, brightness and contrast, foreground and background, and changeable figures.