Science Of Optical Illusions

Dr. S. S. Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106

2017-05-08 03:29:07



Optical illusion

An optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by visually perceivedimages that differ from objective reality. The information gathered by the eye is processed in the brain to give a percept that does not tally with a physical measurement of the stimulus source. Most of us might not have experience but must have heard about occurrence of optical illusions.  Stories of optical illusion faced by Duryodhana inside the famous “palace of illusions” made by Pandavas (in Mahabharata times) are well known to 

everybody.  Although there is no description of the design of the palace constructed by Maya (Architecture) for the Pandavas having many specialties such as highly reflective floors, walls, doors and water ponds were easily mistaken. Nothing in the Palace was really what met the eye.Walls could be walked through and seemingly ‘empty spaces were really walls. A water-pool could actually be walked on and what looked like a safe plank was really water. Confusing state experienced by Duryodhana there in the palace due to optical illusions like seeing water in place of floors, floors in place of water ponds, walls as thorough ways and open spaces as walls lead to Draupadi to make a comment which is said to be the starting point towards great destructive war of that times. However, there was nothing magical about the palace, with an expert arrangement of optical illusions; the palace was engineered to appear magical. Such illusions are created by cleverly incorporating the optical phenomena of reflection and refraction into architecture.  

One more very common optical illusion during Mahabharta battle is described to make sun disappear by Lord Krishna in disguise of thick clouds pretending to be sunset in order to make possible for Arjuna to fulfill his promise to kill Jayadrath before sun set (to avenge the killing of Arjuna’s son Abhimanyu). Similarly, the principle of the pinhole camera is used in the Virupaksha temple and here the light rays going through a tiny orifice and falling on a wall produce an inverted image of the light source on the wall. This method is seen in operation when the image of the gopuram of the temple falls inverted on one of the walls of the main temple 100 feet away. Another well-known optical effect is seen on the face of the Buddha statue in an Ajanta cave: The carved expression on the face indicates different moods depending on the location of external light falling on it. Use of mirrors in lighting the underground chambers or other interiors of the fort in Gwalior is another clever optical arrangement. Likewise, in the fort of Chittorgarh, one can see a demonstration of how Padmini's face was shown reflected to Allauddin Khilji to satisfy his curiosity, without letting him actually see her.

Further, those who have been in touch with books and real life scenes might have read or experience about mirages in deserts, difficulty in driving vehicles with head light on a rainy night, water and smoke like situation on metalled roads during a hot day.  Curiosity to learn about such optical illusions is well within everybody who may understand the science behind them or not.  In the internet era, many examples of optical illusion can be found easily and experienced.

There are three main types: literal optical illusions that create images that are different from the objects that make them, physiological illusions that are the effects of excessive stimulation of a specific type (brightness, color, size, position, tilt, movement), and cognitive illusions, the result of unconscious inferences. Pathological visual illusions arise from a pathological exaggeration in physiological visual perception mechanisms causing the aforementioned types of illusions. Optical illusions are often classified into categories including the physical and the cognitive or perceptual, and contrasted with optical hallucinations. Physiological illusions, such as the after images following bright lights, or adapting stimuli of excessively longer alternating patterns (contingent perceptual after effect), are presumed to be the effects on the eyes or brain of excessive stimulation or interaction with contextual or competing stimuli of a specific type—brightness, color, position, tile, size, movement, etc. The theory is that a stimulus follows its individual dedicated neural path in the early stages of visual processing, and that intense or repetitive activity in that or interaction with active adjoining channels cause a physiological imbalance that alters perception.

Science of optical illusions

To make the optical illusion happen it not the need to be master of Vastu Shastra, Jyotirganita-Surya Siddhanta, Aintiram, cartography, the Brahma principle, the yogashastra, etc. but only we need to understand are the fundamental of physics. To be able to see or not to see an object simply depends on the control of optical phenomena like reflection, refraction, transmission, absorption, total internal reflection, dispersion and polarization of light waves at media interface.  Optical illusions neither trick the eye nor fool the brain, but teach us about our visual perception, and its limitations. Emphasis here is on beauty, interactive experiments, and attempts at explanation of the visual mechanisms involved. Illusion optics is an electromagnetic theory that can change the optical appearance of an object to be exactly like that of another virtual object, i.e. an illusion, such as turning the look of an apple into that of a banana. Invisibility is a special case of illusion optics, which turns objects into illusions of free space. The concept and numerical proof of illusion optics was proposed in 2009 based on transformation optics in the field of meta-materials. It is a scientific disproof of the idiom 'seeing is believing'. Illusion effects do not rely on the direction and form of incident waves; however, due to dispersion limitation of specific material parameters, the functionality of illusion only works in a narrow band of frequency.

Mirage is an optical phenomenon very common in sunny days. It's caused by the redirection of the reflected light rays form the object, in other words, is a real physical phenomenon and not just an optical illusion. The sun light in the direction of the road gets refracted do to the temperature gradient (continuous change) of the layers of air. This refraction causes the redirection of the sun rays and finally it gets reflected (total reflection) from the layers of air near the surface of the road. This phenomenon can also be observed when the road surface is very cold and in this case the reflected images are upside down. The phenomenon of  Mirage is the dependence of the refractive index of air on the density of air, which, in turn, depends on the temperature of the air (hot air being less dense than cold air). While driving down the road on a summer day and looking ahead toward the horizon, one notice that the road looks like there's a puddle of water on it, or that it was somehow wet. Of course, on getting closer, the effect disappears. A non-constant density leads to refraction of light. If there's a continuous gradient in the density, we get a bent curve as opposed to light coming straight. The eye does not know, of course, that the light coming at it was bent, so the eye/brain continues the incoming light in a straight line. This mirroring of the car (or other objects) then tricks into thinking the road is wet, because a wet street would also lead to a reflection. In addition, the air wobbles (i.e. density fluctuations), causing the mirror image to wobble as well, which adds to the illusion of water.

Some examples of optical illusions (on internet)

Architects are continuing to push the boundaries of what's possible when it comes to three-dimensional structures, and these illusory architectural designs are some mesmerizing examples of how the use of optical illusions can create a mind-boggling appearance. From buildings that appear to be floating above land to psychedelic interior spaces, these illusory architectural designs are sure to have people visually consumed by the variety of optical illusions. Optical Illusions can use colour, light and patterns to create images that can be misleading to our brains. The information gathered by the eye is processed by the brain, creating a perception that in reality, does not match the true image. Optical illusions simply trick our brains into seeing things which may or may not be real. Optical illusions are popular again thanks to the internet. Here are some classic optical illusions available on the internet with all explanation why they continue to fool people to this day.


·         Black on White

·         Blivet

·         Blue vs Green

·         Cafe Illusion

·         Checker Shadow Illusion

·         Endless Staircase

·         Fading Image

·         Floating Stairs

·         Fraser Spiral

·         Hermann Grid (The Scintillating Grid Illusion)

·         Jastrow Illusion

·         Kanizsa Triangle

·         Leaning Tower

·         Lilac Chaser

·         Mach Bands Illusion

·         Monster Illusion

·         Ponzo Illusion

·         Rotating Rings

·         Rotating Squares

·         Scintillating Grid

·         Spinning Silhouette

·         Static Motion

·         Titchener Circles

·         Up and Down

·         Wonder Block

·         Zöllner Illusion