In image formation, we have already mentioned that we "see" with our brain, not just with our eyes. When an image falls onto the retina, nerve impulses are sent to the brain where the actual perception takes place. The brain pieces together bits and pieces of information, matches the retinal image with the images stored in our visual memory, so that we know what the object is, and interpret visual clues so that we know where the object is.

However, there are times when the visual stimuli received cause the brain to make a wrong interpretation, so that what we see is different from the actual reality. This erroneous perception is called a visual illusion.

There are many types of optical illusions: some are everyday natural phenomenon, some are created for psychologists to interpret how our visual system works, some are used for artistic and architectural purposes, and others provide pure visual pleasure.

The following are selections from famous yet simple visual illusions. Explanations are provided for some of them, So enjoy!

1. Influence of background / surrounding objects

Is that a square you see in the diagram?
The curved lines make the straight edges of the square curved by contrast.

The two lines should be parallel. Or should they?

Which fan is larger?

2. Impossible figures

Based on our experiences in life, many objects we come across are three-dimensional. So on looking at these two 2-D diagrams, our brain tends to interpret them as 3-D.

Also, when we look at a picture, our eyes move constantly and take quick glimpses of the more important parts of the picture to look for visual information ---- fixation.

These images are then successively formed onto the retina's fovea, where the visual acuity is the greatest and the images are seen most detailedly. These separate information then correlate with each other to form a single picture in our brain.

However, on looking at these two diagrams, the information extracted from the quick glimpses cannot form a sensible 3-D diagram, hence the name --- impossible figures.

But if these diagrams were to be reduced in size such that the whole image can be focused on the fovea, we would just see them as pure 2-D line images.

3. Ambiguities

We term these pictures as ambiguities because they can be perceived in more than one way.

Figure-Ground ambiguities

When we look at a picture, we subconsciously divide it into figure (the main object) and ground (the background). The figure will appear to stand more out than the ground.

• If you regard white as the figure, you'll see a vase.
• If you regard black as the figure, you'll see two persons face to face.

How we choose which part of the picture as figure is not a totally random decision. If you were shown pictures of unambiguous young women beforehand, you'll also expect to see a young woman in the above picture. This is an illustration of a top-down process way of thinking.

Object ambiguities

You can perceive the Necker cube in two ways, the shaded side as the top or the inner side of the cube.

4. Size and Depth perception

Which horizontal line appears longer?

Does the top vertical part appear longer than the bottom part? Use a ruler to help you.

Although the objects we look at may be 3-D, the images formed on our retina are 2-D. For the brain to interpret them as 3-D, it look for depth cues. One such cue is relative size. A very distant object will look smaller. Another cue is the convergence of non-parallel lines, which indicates increasing distance from us. (e.g. the convergence of railway tracks into the distance.)

To explain the illusion of the length of the lines above, you need to know about size constancy:

Objects do not change much in size as we move near or away from them. Their perceived size remains relatively constant unless the distance change is quite drastic.

When we move near an object, the image size on the retina (retinal image size) increases. However, we can also perceive the distance of the object to us as shorter.

Emmert (1881) suggested the following :
perceived image size (in brain) = (retinal image size) x (perceived distance)

In , the converging non-parallel lines indicate increasing distance. Line A's perceived distance is longer, but the retinal images of both lines A and B are the same. By Emmert's theory, the perceived size of A is larger (longer).

In , you may imagine it to be a line protruding to you. Perceived distance is shorter, retinal image being the same, perceived length will be smaller.

The illusion of Ames room can also be explained using Emmert theory.

5. Colour and Brightness illusions

In the above set of black squares, you'll see shades of grey at the intersections. This is because the contrast between black and white is great and lateral antagonism exhibits its effects. However, when when you focus specifically on one intersection, the shades of grey in that spot will disappear.

7. Daily life illusions

You are also facing visual illusions in your daily life:

a) TV ----- All the colours you see on TV are just due to 3 colours (red, green and blue). If you look close enough, you can only see many closely packed dots of 3 colours. Because they are so close, the retinal images overlap and different colours result.

b) The bent spoon in your cup of water and the apparently shallow swimming pool ----- are due to refraction i.e. light travelling with different speeds in different media.

c) Clothes with vertical stripes make a person look thinner than clothes with horizontal stripes.

d) The moon racing through the clouds ----- we tend to view large objects (the large clouds) as stationary and the smaller object (the moon) as the one moving.

e) A red car looks larger than a green car of the same model when viewed from far above, because of different speeds of light.