Normal color vision is dependent on the presence and function of three types of cones, which detect photons of low, intermediate, and high energy levels. If any of the three types of cones are missing or non-functional, the full range of color cannot be perceived.
An inability to perceive the normal range of colors is called a color deficiency. A sex-linked trait, color deficiency occurs more often in males than females. There are many levels of color deficiency and a variety of causes.
Monochromats are totally color-blind. In this category, individuals may have all three types of cones missing or defective, or they may have only one type of functioning cone. They are unable to distinguish one color from another. This would be analogous to watching a color television set, but only seeing the image in black and white. This level of color deficiency is extremely rare.
Dichromats may be red-green or blue-yellow color deficient, but the latter is quite rare. In red-green color deficiency, the individual cannot readily distinguish between red and green light. This could be due to missing or defective cones that respond to green light. The individual sees colors as if only the red and blue cones are functioning.
Trichromats are receptive to all three primary colors of light and can perceive the full normal visible spectrum; but even in this category, individuals may differ in their ability to perceive and distinguish color variations.
An optical illusion occurs when what we see does not correspond to physical reality. When an expert magician performs disappearing coin tricks or when a stick appears to bend as it enters the water, we are reminded of how easily we can be tricked by what we see. The root of the word illusion is the Latin verb illudere meaning, "to mock." The viewer is quite literally mocked into seeing something that is different from what is actually there. There are many kinds of optical illusions. Some examples are illusions of size, shape, direction, motion, and perspective.
Probably one of the most common geometric optical illusions is the one at the right. This 2-dimensional illusion is known as the Mueller-Lyer illusion.
Which line is longer? Even when the viewer is familiar with the above illusion, it is easy to be fooled by the apparent length of each segment.
Below is an illusion of depth created with the Mueller-Lyer model. Depending on how the viewer sees perspective, the vertical line of the room may appear farther away than the vertical line of the building. Although the vertical lines produce equal retinal images, one seems closer to the viewer than the other does. The size illusion of the Mueller-Lyer illusion is still at play. Because of depth cues, we "know" that objects that are further away appear smaller. The building seems farther way because it is perceived as being smaller than the room.
The Ebbinghaus illusion is an illusion of area. The circle that is surrounded by smaller circles appears larger than the circle surrounded by the larger circles. Below is an example of an illusion of shape. What do you see
Causes of optical illusions
What happens when we experience optical illusions is not entirely known. The answers we look for in order to explain the phenomena are found among the studies of optics and psychology. Optical illusions are caused by the eye itself, the brain's interpretation of the visual information, or in many cases, a combination of the two. Light reflecting off of an object enters the eye through the cornea in order to form the image that is projected onto the retina. After passing through the cornea and lens, the retinal image may be distorted. This distortion can be responsible for optical illusions. Reduction in the sharpness of the image or the distortion caused by the shape of the retina can both be contributing factors depending on the illusion. Other factors that can be responsible for optical illusions involve the ways in which the brain interprets visual images. These factors can include depth perception, relative size perception, or the misapplication of past experiences in visual interpretation. Individual properties that can influence these factors involve the viewer's age, personal experience, and cultural /environmental factors.
An in-depth look at one illusion
In experiments, scientists blurred the Mueller-Lyer illusion (see right) in order to show that blurring increases the strength of the illusion, an example of how our eye can affect how we see. It has also been shown that the strength of the Mueller-Lyer illusion decreases with age, as a result of experience. In yet another theory to explain this illusion, scientists hypothesized that the length of the eye movement in order to scan each segment could result in the perception that one segment is longer than the other is. Because the outward "V" of the illusion lengthens the eye movement, we perceive that segment as longer. Principles are yet to be found that are applicable to most optical illusions. Optical illusions are usually caused by more than one physiological or cognitive factor and in most cases they are usually the result of an interrelationship of a number of them. The eye receives the images, but the brain interprets them. The result is sometimes that we see what we think we should see rather than the physical reality of the object. Although our overall experience teaches us to trust our senses, optical illusions remind us that what we see is also a product of our minds.
Near-sightedness (myopia), as the name suggests, comes from the fact that individuals with this condition are generally able to see near objects without any corrective lenses; however, they need correction to see objects at a distance. One reason people may be near-sighted is that the eyeball is actually too long, so that the lens focuses the image in front of the retina. To correct for this condition, the optometrist prescribes glasses which disperse or spread the light just before it enters the eye. These lenses are thinner in the middle than the outside.
Far-sightedness (hyperopia) is a condition in which people are able to see at a distance without correction, but not close up. One reason this occurs is if the eyeball is actually too short for the lens to focus the image properly on the retina. The corrective lens prescribed will converge the incoming light to assist the eye’s own lens; these glasses are thicker in the middle than on the outer edge. As people age, their lenses become less elastic and are not able to thicken sufficiently to read up close; thus reading glasses, thicker in the center, are helpful.