Science behind Optical Illusions – An Explanation with the help of Visuals

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The brain is set up to receive and interpret messages from the eye. Optics, a branch of physics, studies the interaction of the light and the eye and this interaction plays an important role in optical illusions. Optical illusions use light, colors and other features to trick the mind.

There are various reasons due to which we see an optical illusion and foremost among them is the physical one. On physical bases, an afterimage is an apt example of optical illusion. Afterimages occur when you look at bright colors for too long. After the image is gone, you can still see imprints of the image. Another factor that plays its influence is habit. The brain likes seeing the same objects over and over again and even when it sees something out of the ordinary, it reinterprets this strange image as something more familiar, regardless of what the real image is. The feature covers the physics related part of optical illusions with their respective animations.

1) Mach Bands Illusion

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Named after Ernst Mach, this optical illusion enhances the contrast between the slightly different shades of grey as they combine with each other. As you can see in the animation that the three blocks look quite similar when they are apart, but upon joining the boundaries, the contrast becomes quite exaggerated. The exaggeration is due to the light reflected from each of the bands which overlaps the boundaries of the bands. The differing bands lead the visual system to believe that different luminance’s are present which results in a gradient when the edges of the bands are connected. The illusion is mainly a result of the human visual system’s filtering mechanism which is carried out on the luminance channel of the image captured by the retina. This illusion is of major importance in evaluating dental radiographs for decay analysis. In dental radiographs, gray scale images of teeth and bone are analyzed for abnormal variances of intensity.

2) Lilac Chaser Illusion

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In this optical illusion, the viewer sees a series of lilac colored blurry dots arranged in a circle around a focal point. As one stares at the focal point, it will appear as if a space is running around the circle of lilac discs but after about 10 to 20 seconds, the viewer will then see a green disc moving around the circle instead of the space. A longer inspection will lead to the disappearance of the lilac discs and you will only see the green disc moving around in a circle. This optical phenomenon is due to a number of reasons. Firstly, we perceive movement due beta movement since when we see something at one point and then at another which leads us to believe that it is in motion. Secondly, the appearance of green disc is a due to an afterimage result by which we see a green after image in place of the lilac discs and lastly, the eventual disappearance of the lilac discs is due to Troxler fading, which occurs when blurry objects that are located in the periphery of our visual field disappear while we have our eyes fixated on a certain spot.

3) Ponzo Illusion

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In the Ponzo illusion, two identically-sized lines appear to be different sizes when placed over parallel lines that seem to converge as they recede into the distance. Demonstrated in 1913, the illusion works due to the reason that we interpret the scene in a linear perspective. The vertical lines seem to grow closer to each other as they move forward due to which we perceive that the top line is much further. We know that an object in the distance would need to be longer in order for it to appear the same size as a near object, so the top far line is seen as being longer than the bottom near line, even though they are the same size as you can see in the animation.

4) Hermann Grid (The Scintillating Grid Illusion)

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This optical illusion, named after the physiologist who discovered it, shows that the white dots at the center of each square shift from white to grey. You will notice that as you focus your attention on a specific square, you will notice white dots at its corners but as soon as you shift your focus to another area in the image, the white dots will change to grey color. Consider two regions of the retina in which one region views an intersection of a white horizontal and vertical band, while the other views a white band between two intersections. Though the two regions themselves receive the same amount of light, the situation in their neighboring regions is different. At the intersection, light comes in from all four sides, but the white band that lies between the two intersections is surrounded by two dark sides. This leads to a phenomena called lateral inhibition, which causes a bright surround to an area appear darker and, conversely, a dark surround to an area appear lighter.

5) Spinning Dancer Illusion

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Also known as silhouette illusion, this optical illusion resembles a pirouetting female dancer who abruptly changes direction if you view her closely. It may seem that see is spinning in one direction only but constant observation may reveal that she is spinning both in clockwise and anti-clockwise direction. This illusion is mainly related to bistable perception in which an ambiguous 2 dimensional figure can be seen from two different perspectives. The absence of third dimension allows the brain to construct space around the figure which makes the illusion work. A similar kind of illusion is the Vase/Face illusion.

6) Checker Shadow Illusion

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Created by Edward Adelson, this optical illusion shows that the area labeled A appears to be darker than the area labled B whereas both the areas are of same color. While we attempt to determine the color of a surface, our brains know that shadows are misleading and that they make surfaces look darker than they normally are so we compensate by interpreting shadowy surfaces as being lighter than they technically appear to the eye. This is where the illusion plays its part and thus we infer that the square B is lighter as compared to square A but in reality square B is just as dark as square A as demonstrated in the animation.

30075   25/09/2014

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