Literal illusions, physiological illusions, and cognitive illusions are the three basic forms of optical illusions. Literal illusions include images that appear to distort reality; for example, a mirage is an illusion that causes you to believe you are about to fall into a body of water when in fact there is no water nearby. Physiological illusions occur when your eye is able to detect physical differences on something that cannot be seen by the human eye; for example, when looking at a scene through a glass window, you might see a double image because your eyes are unable to distinguish between the reflection from inside the house and the actual object outside the window. Cognitive illusions involve incorrect assumptions that you make about the world; for example, assuming that shapes that are close together look like one another only because they are being viewed out of sequence. There are many more types of optical illusions, but these are the most common ones.
Cognitive illusions are caused by our preconceived notions and assumptions about the environment, which we apply to visual inputs. This can result in four different forms of cognitive illusions: ambiguous illusions, distorting/geometrical-optical illusions, paradox illusions, and fictions (image source).
Ambiguous illusions occur when our brain interprets two conflicting stimuli as one clear image. For example, a linearly expanding ball appears to be flattened out on a table when viewed from above but not from below. The visual system combines information from both views to create a clear image of a three-dimensional object. Ambiguous images may also arise when our eyes report seeing something that isn't really there, such as in the case of "phantom limbs". Our brains fill in the gaps between what our eyes tell it and what other parts of the body are doing so that we can make sense of the world around us.
Distorting/geometrical-optical illusions involve our brain applying rules it has learned over time to visual inputs, which can lead to false conclusions being drawn. An example of this is the common optical illusion known as "the ladder", which most people perceive as two separate lines connecting two distant points instead of one. The apparent separation of these lines occurs because we assume geometry behind the appearance of objects and therefore draw conclusions based on this assumption.
Illusions affect one's perceptions. The majority of illusions fool the eyes, hearing, and skin, however other illusions may affect perception owing to changes in interior body components. Optical illusions, auditory illusions, and tactile illusions are the three basic forms of illusions. Optical illusions include moving objects, gridlines, and contorted figures. Moving objects illusion: When viewing a scene that contains moving objects (for example, a car driving by), an observer may see it as though the objects were still when in fact they are not. The human brain performs this trick by recognizing identical patterns even though they are in different places. This illusion does not harm anyone who sees it, but it can be dangerous for drivers in heavy traffic areas or people who work with tools that rely on visual inspection. Gridlines illusion: When reading a map or other image containing gridlines, an observer may think there is no such thing as topography when in fact there is. The human brain knows better than to trust its own senses in these circumstances and will often override them with information from other sources. For example, an observer may see a straight line on a map that appears to go off the edge of the paper, but this is only an illusion caused by water dripping on the map from a leaky tap. Contorted figures: An observer may see a figure of eight even though it is actually a square. No wonder we need our brains!
Optical illusions, often known as visual illusions, are forms of visual trickery. A broad range of deceptive visual illusions can be perceived due to the arrangement of pictures, the influence of colors, the impact of light sources, or other variables. Some folks just cannot notice the effect of some illusions. Others may find it amusing to see what will happen when they put different objects in their line of sight.
There are several categories of illusion: geometric, motion, color, texture, depth, size, and orientation. Many illusions have been identified by scientists as having a specific physical cause. For example, the classic Mooney-Rivlin face illusion is caused by an interaction between surface tension and image structure that results in one region of space being pulled down by its own image.
People have been creating illusions since we first started making pictures. Early artists such as Giotto and Da Vinci used intuition rather than scientific understanding to create their works. As science has advanced so has our ability to manipulate reality. Modern scientists use computers to simulate natural phenomena and try out new ideas before building actual models or doing experiments. They have created many beautiful illusions using computer graphics.
Illusions are interesting because they show us something about how our brains work. The fact that we often don't realize that two slightly different images make up one single picture shows that our brain has some special mechanisms to combine information from different sources.