Optical Illusions

What are illusions? Illusions trick us into perceiving something differently than it actually exists, so what we see does not correspond to physical reality. Hence, the word illusion comes from the Latin verb illudere meaning, "to mock." In addition, some illusions show us one thing in a picture, while someone else sees something entirely different in the same picture.

Optical Illusions can use colour, light and patterns to create images that can be deceptive or 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. Perception refers to the interpretation of what we take in through our eyes. Optical illusions occur because our brain is trying to interpret what we see and make sense of the world around us. Optical illusions simply trick our brains into seeing things which may or may not be real.

Try out some of these illusions and discover just how tricky it can be for your brain to accurately interpret the images from your eyes.

Mirages That Play Tricks on the Eyes

Mirages are nature's version of optical illusions. Variables like the path of light particles, the curvature of the Earth and air temperature can create false images that the eye is convinced are real. Mirages are the subject of many legends. So-called Fata Morganas, which make land and ships appear like they are floating in the air above the sea, have been unnerving sailors for centuries, while mirages involving oases have given false hope to many thirsty desert travellers.

Scientifically, most mirages can be explained by the fact that photons (particles of light) move faster through warm air than through cooler air because hot air is not as dense. This is why mirages are common in deserts, oceans and other places with hot or extremely varied temperatures.

Fata Morgana

For superstitious sailors, the phenomenon known as the Fata Morgana must be terrifying. The illusion occurs on oceans and seas, and it makes distant objects such as other ships or shoreline appear to be floating in the sky. Some even attribute the legend of the ghostly Flying Dutchman ship to Fata Morgana mirages. This is not only an ocean phenomenon; there are accounts of such illusions on the big lakes as well.

Like most mirages, Fata Morganas — named after Morgan le Fay, the sorceress in the legend of King Arthur — appear when the light is refracted (or "bent") by contrasting air temperatures. In oceans and seas, the air near the surface is sometimes cooled by the water, so the temperature is warmer at higher altitudes. Light passes through hot air more easily, so it reaches the eyes of a far-off viewer after refracting above the cooler air. The viewer’s brain expects that light travels in a straight line, so it's fooled by the refraction and perceives that the far-off object is floating above the water.

A Fata Morgana seen from the coast of Queensland

Real Footage of Fata Morgana

Fake! Footage of Fata Morgana

Desert Mirage

Like Fata Morganas, desert mirages occur because light bends to move through warmer, less dense air. In the desert, refraction-caused illusions are known as inferior mirages (as opposed to Fata Morganas, which are superior mirages). “Superior” and “inferior” refer to where the mirage takes place. Superior means it's above the horizon, while inferior means it's below. This is why inferior desert mirages usually show up as water-like images on the ground.

In the desert, the air is at its hottest near the surface, and it cools as it rises. This is why the light refracts downward, causing the eye to see sky-like (or water-like) colours below the horizon. A similar illusion is very common on hot highway pavement. You have probably noticed that the road often appears wet or covered with puddles in the distance on an especially sunny day. This is caused by the same phenomenon that creates fake desert oases.

Brocken Spectre

The Brocken spectre, named after the highest peak in Germany’s Harz Mountains, has the potential to be the spookiest mirage that a person will ever encounter. Mountain climbers were the first to experience this visual phenomenon. They were confronted with ghostly human-like figures apparently looking at them through the high-altitude haze. In reality, people who see a Brocken spectre have no need to feel frightened because they are seeing their own shadow.

The spectre occurs when the sun is behind the observer. The light casts a shadow, not on the ground, but on clouds or fog that occur most often at high altitude. The sunlight that shines around the observer creates a halo-like glow. When the clouds move, the figure may appear to move as well. This phenomenon requires a bright light source shining at a low angle, so it can occur at ground level on foggy days with a strong artificial light such as the “high beams” of a car’s headlights.

A Brocken spectre within a glory's rings
A semi-artificial Brocken spectre created by standing in front of the headlight of a car, on a foggy night.
Shadow of an airplane cast by the sun on nearby clouds

Omega Sun

Omega suns appear to make the shape of their namesake Greek letter when they are just above the horizon. The legs (bottom) of the omega are created by warm water heating cooler air just above the surface. The omega shape can be quite pronounced if the water is calm.

Like other ocean horizon mirages, omega suns are caused by light refracting through warmer air (in this case, near the surface of the water). Because the water, especially in an ocean, sea or large lake, is more constant than the air when it comes to temperature, omega suns are common in colder climates during the winter. In some places, such as Japan, seeing an omega sun is considered a good omen.

The Moiré Effect

The following pictures are NOT animated. Your eyes are making them move. To test this, stare at one spot in each picture for a few seconds and everything will stop moving; OR look at the black centre of each circle in the first picture, and it will stop moving; but when you move your eyes to the next black centre, the previous one will move after you take your eyes away from it.

Adelson's Checker-Shadow Illusion

Look at the two target squares labelled "A" and "B". Is one darker than the other? Hover your cursor over the image to reveal the truth.

The tile labelled "A" appears significantly darker than the tile labelled "B". But in fact they are both the same shade of grey.

Hermann Grid Illusion

Count all the black dots you can see.

There are no black dots! If you focus directly on each dot, you'll see that all of them are white. 

Scintillating Grid

Allow your gaze to roam around the figure. Focus at one of the intersection points from time to time.

Dots which are not centred in your visual field should appear to flash from black to white, in a 'scintillating' effect. If you focus on one of the dots it will appear constantly white.

Müller-Lyer Illusion

Look at the two horizontal lines; note the apparent difference in length difference. The top horizontal line (with arrow heads pointing outwards) should appear to be longer than the bottom horizontal line (with arrowheads pointing inwards). However, they are in fact the same length.

Ebbinghaus Illusion

The orange disc on the right appears larger than the one on the left, but both discs are precisely the same size.

Café Wall Illusion

This illusion consists of a checkerboard-like arrangement, in which individual rows have been displaced. Look at the grey horizontal lines between the rows of the board. Are they parallel to one another? The horizontal grey lines appear to be angled with respect to one another - in fact they are all parallel.

Ponzo Illusion

Look at the two yellow lines and note how the higher one appears longer than the lower one. The higher up yellow line will appear longer the lower down yellow line (when in fact it is not).

Colour Saturation Illusion

Within the larger grey and blue squares are two smaller squares. The small square on the left appears to be a blue or violet colour while the small square on the right appears to grey. They are, in fact, the same color.

By making the background surrounding each smaller square the same, it is obvious that they are the same colour.

Rotating Dot Illusion

If your eyes follow the movement of the rotating pink dot, the dots will remain only one colour, pink. However, if you stare at the black "+" in the centre, the moving dots turns to green.

Now, concentrate on the black + in the centre of the picture. After a short period, all the pink dots will slowly disappear, and you will only see a single green dot rotating.

It's amazing how our brain works. There really is no green dot, and the pink ones really don't disappear.

Rotating Flower Illusion

Each coloured dot continuously moves back and forth in a straight line. However, when all the dots are viewed as a whole, our brains perceive a rotating flower.

Concentrate on one specific colour dot at the time!

Ninio and Stevens Illusion

There are a total of 12 black dots in this image, but you can't see them all at once. While you should be able to see any dot you look at directly, the dots in your peripheral vision seem to appear and disappear.

Jastrow Illusion

The illusion also occurs in the real world. In the illustration below, the two toy railway tracks are identical, although the lower one appears to be larger.

The Spinning Dancer Illusion

The spinning dancer illusion is an example of a bistable motion illusion. It is possible to see the dancer moving either clockwise or anticlockwise. Often the direction of movement will suddenly switch as you are watching the image.

It has been suggested that the direction of perceived movement may depend on which hemisphere of the brain is more active.

Motion after-effect Illusion

Waterfall illusion, or motion after effect, is an illusion of movement. It is experienced after watching a stimulus moving in one direction for some time, and then looking at a stationary scene. The stationary scene appears to have movement (in the opposite direction to the moving stimulus that one previously watched). This is called the “waterfall illusion”, as it can be experienced after watching the motion of the water in a waterfall, and then attending to a stationary scene, for example the rocks by the side of the waterfall.

More complex version of motion after effect illusion.

Anaglyph 3D

The first 3-D film, "The Power of Love," screened at the Los Angeles Ambassador Hotel in 1922. To create the 3-D effect, two films were projected simultaneously while the audience watched through stereoscopes.

3-D films didn't reach mass appeal until the 1950s, mostly with the cult and horror genres. This is when the white cardboard glasses with red and blue lenses came into vogue -- the technical name is anaglyph. Despite anaglyph glasses' universal association with 3-D, they're rarely used any more.

The traditional 3-D anaglyph glasses use one red lens and one blue (cyan) lens. Many other colour combinations work, too, like red and green, but red and blue were used most frequently. The blue lens filters out all the red light, and the red lens filters out all the blue light, so each eye sees a slightly different image.

When the 3-D film is projected on the screen, two images are displayed: one in red, one in blue. Since each lens of the glasses has a filter, only one image can reach each eye. The brain merges both images together, which results in the illusion of an image popping or jumping off the screen.

However, using the red and blue colour combination to get a sensation of depth also causes a degradation of colour in the film. The image quality is a bit fuzzy, and the color of the 3-D film always looks a little off. The industry responded with polarized 3-D lenses to create a better experience for moviegoers.

Polarized 3-D glasses don't rely on separately coloured lenses to get the 3-D effect. They work a lot like anaglyph glasses, though, by tricking the brain into merging two images into one. Polarized lenses have a yellow-brown tint instead of red and blue. When light is polarized, all the waves oscillate in one direction. The camera projects one image that polarizes vertically, and another polarizes horizontally. Each lens only lets through one kind of image, and your brain merges each image into one. The end result? Crisp 3-D images with very little degradation in colour or quality!

3D Images

3D Films Links