Visual illusions
Optical illusions they are visual perceptions ascribing property to the viewed object, which are not really there. The causes of optical illusions are generally not known yet; only known, with the psychological factor playing a decisive role. Most of the optical phenomena are related to this. that the eye does not perceive images of individual objects (or fragments thereof) independently of each other, but in conjunction with other objects around him (or fragments of an item). Let us consider a few examples, which will help us understand the formation of visual illusions occurring in the group of optical phenomena.
Rys. a. Episodes ending with arrows.
Rys. b. Division of a segment into two equal parts, and a division of a half of the segment into 16 equal parts.
Figure a shows two segments ending with arrows. The top segment has arrows pointing outward, the lower one – inwards. When we look at this drawing, it seems to us, that the top line is longer than the bottom line. We check with a measure, whether there was a mistake in measuring the lengths of the sections and we find out, that they are geometrically equal, against apparent visual illusion
The second example, Fig. B, in which two episodes have been divided into two equal parts, but the arrangement of the marked arrows is different, confirms the occurrence of apparent differences in the length of the sections.
This phenomenon, called the Muller-Lyer illusion (Georg Muller – German scholar, one of the founders of experimental psychology dealt with, together with Lyer, mainly the study of visual impressions and the conditions for the formation of associations), can be explained as follows. When we look at both episodes, it is impossible to separate them from the arrows. In the case of the arrows pointing outwards, we see a segment with dimensions much longer than in the case of the arrows pointing inwards.
A similar example of visual illusions is shown in Fig. b, in which the section of the horizontal straight line is divided into two equal parts, one of which was divided into 16 equal parts. This divided half of the straight line seems apparently longer than the undivided one.
By drawing parallel, vertical or horizontal lines and dividing them into several parts with short segments inclined at the same angle, but in reverse directions (fig. 3), we create an image seemingly inconsistent with the construction of the drawing. It seems to us, that episode a,, being the distance between two vertical lines, is greater than the same segment a,. Also in this case, we get an apparent image, as if vertical or horizontal segments in such an approach were not parallel to each other, but slightly slanted. This phenomenon is called the Zöller formula.
Two horizontal parallel lines AB and CD were intersected by rays from point O. We have an impression, that these parallel lines are curved outward (rys. 4). In the drawing 5 the same parallel lines are shown, but intersected by rays derived from points O and O1. In this case, the reverse phenomenon of the visual illusion occurs, as if the lines were curved inward. A similar example of a visual illusion is shown in the drawing 6. The given drawing examples concern visual illusions occurring in various systems of sections.
Let us move on to other examples of visual illusions found in geometric figures, so on the planes. We draw two squares very precisely (one inside the other) with sides parallel and equidistant from each other, followed by a series of dashes, that will intersect the sides of the squares, as shown in the figure 7. When we look at this drawing it seems to us, that the geometric square was bent. The same visual illusion is obtained by crossing the circle with a different arrangement of segments (fig. 8). This circle appears crooked, even though it was correctly drawn with a compass.