Optical and Laser Illusions

Rajesh Uppal January 30, 2023 BioScience, Photonics Comments Off on Optical and Laser Illusions 232 Views

Optical illusions (or visual illusions) are tricks that your mind plays on you based on what you think you are seeing. The image that is being visually perceived is deceptive and does not tally with actual reality. So what you are seeing and what comes out after being processed by the brain does not fit or agree with each other.

When an image is focused onto the retina, bio-electrial signals are transmitted via the optic nerve to the visual cortex of the brain for processing. The visual cortex takes this information (visual stimulus) from the retina and matches it with previous images stored in your memory, in order to interpret to interpret what you are seeing. This process is reliant on previous experiences as well as other visual clues.

However, sometimes the combination of various visual stimuli can confuse the visual cortex. This causes the brain to interpret the images wrongly, and hence, what you see is not the same as what it is in actual reality. This is how optical illusions come about.

There are many different types of optical illusions. Some are common illusions that occur naturally, some are specifically designed by psychologists to test the visual processing system, while others are created as works of art.

In general, optical illusions can be classed into 3 main types:
– Literal or physical illusions, such as mirages and rainbows. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. A classical example for a physical distortion would be the apparent bending of a stick half immersed in water. A familiar phenomenon and example for a physical visual illusion is when mountains appear to be much nearer in clear weather with low humidity (Foehn) than they are. This is because haze is a cue for depth perception, signalling the distance of far-away objects
– Physiological illusions, which occur from prolonged specific stimulation, such as brightness, tilt and movement. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. an example for a physiological paradox is the motion aftereffect (where, despite the movement, the position remains unchanged).
– Cognitive illusions, which arise from subconscious interference. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known. An example for physiological fiction is an afterimage.

And in each class, there are four kinds: Ambiguities, distortions, paradoxes, and fiction. Pathological visual illusions arise from pathological changes in the physiological visual perception mechanisms causing the aforementioned types of illusions

Optical illusion on Battlefield

Flashing bright lights have been used to dazzle adversaries throughout the past century. In World War II, the British mounted carbon-arc searchlights on tanks as a means of blinding Nazi pilots attacking ships on the Suez Canal. The system, called the Canal Defense Light, shot a flickering bright light—which the developers thought was especially disorienting—through a turret slit, aimed at attacking aircraft. It owed its blinding effect to rapid-fire bursts of activity from neurons within the retina and the first several stages of the brain’s visual system that respond to lights switching on or off (as we have shown in our own research). Although the tanks were deployed to the canal to deter bombing runs, they were not used.

Today military vehicle and weapons decoys are highly realistic and can go unrecognized to within a few hundred yards. They can be deployed and removed within minutes. This type of mimicry works because the human visual system has limited acuity and thus resolves details of shape as a function of distance (the closer you are, the more detail you see). Decoys are designed with specific minimal viewing distances (and satellite-imaging resolutions) in mind so that analysts cannot easily distinguish the decoy from the real thing. Decoys are much cheaper to make than real weapons. Their strategic use can therefore boost a military’s apparent capabilities at a lower cost.

Pepper’s Ghost

A popular magician’s trick is called “Pepper’s Ghost”. Light will bounce off of clear flat glass as well as pass through it. If the light is just right, you can see things reflected in the glass as well as things on the other side of the glass. If two objects are placed on opposite sides of the glass so the are right where the reflected image of the other is, what you will see is both objects together, apparently in the same place. You can use this to make an unlit candle appear to be lit, or to even burn under water. If you start with one of the objects in darkness, and then shine a light on it, its reflection will “magically” appear in the glass. This trick is used many times in the “Haunted Mansion” in Disneyland and Disney World to make “ghosts” appear and disappear.

Light Bends

When light goes from one material to another, it bends. A laser beam that hits water, glass, or plastic will change direction. Under the water, the beam will look like it came from somewhere else. Light coming up out of water will also bend, making it appear to come from somewhere else. This is why straight sticks will appear to be bent when stuck into water, why fish appear to be farther from shore, and closer to the surface than they really are, and why streams do not look as deep as they really are. If you shine a light straight down at water, most of the light will go into the water, and some will reflect. The same thing happens if light is coming out of water into air. However, if the light inside the water hits the air at larger and larger angles, more and more of the light is bounced, or reflected, back into the water. If the angle is large enough, all of the light is bounced back, and none of it will go into the air. This also happens with clear plastic, glass, and even Jello. We can use this to make “light pipes” that can carry signals in the form of light over very long distances with very little loss of signal.

Water Stream Light Pipe

Water with a little Pine-Sol will scatter enough light to make a laser beam visible. If a laser beam enters a glass bottle just right so that it leaves through a hole, the laser light will bounce around inside of the water stream coming out the hole without leaving the water. You can see the laser beam inside the water if there is enough Pine-Sol, or if the water stream hits something.

Fiber Optic Toys

Many colorful toys and lights use light pipes made of glass, called optical fibers. Fiber optic cables are used to send phone and tv signals in the form of light over long distances.

Ghost Wand

Wave a white rod or board back and forth through the beam from a slide or video projector very fast. It will look like the whole image or screen even though only a bit of the image that hits the rod or board can be seen at any instant.

Persistence of Vision with Lasers

It takes time for our brains to understand what our eyes see. Because of that, a fast moving laser beam spot will not look like a spot at all, but like a line of light. If a laser beam is moved fast enough, the beam spot on a wall can be made to look like a circle or some other figure. However, if you took a photograph of it, and if the camera is fast enough, it will clearly show just a spot. Movies and TV are just a series of still pictures flashed quickly on the screen. The flashing is fast enough so that it looks like smooth motion to us.

Inverted Mask

An inverted, or inside-out mask of a face looks very strange to us. If we move past it, or the mask moves by us, it will appear to turn its head to look at us. This is because of “paralax”. Just like how distant trees appear to move by slower than closer trees as we drive through a forest, the nose of the mask will not appear to move as much, because it is farther away. The sides of the face on the mask will appear to move more because they are closer. However, our brain is very good at seeing faces, even where there are none, such is in clouds. Your brain is easily tricked into seeing the inside-out mask as a face, with its nose closer to us and the ears farther away. The only way your brain can match this with the relatively slow turning nose and more rapidly turning ears is that the face must be turning to follow you. This trick is also used in the “Haunted Mansion” in Disneyland and Disney World.

The Nature of Color

Red objects are red because they reflect red light and absorb, or soak up, all other colors. Blue objects are blue because they reflect blue light and absorb all other colors. If you only shine blue light on a red object, it will appear black, and if you shine only red light on a blue object it will also appear black. This also works with the other colors. If the color of the light shining on an object with many bright colors is quickly changed, it can look pretty freaky.

Color Addition

If you look very closely (think face-plant) at a the image on a TV or computer screen, you will see red, green, and blue spots very close together, especially where the image is white. All colors we can see can be made by mixing red, green, and blue in the right combinations. An equal mixture of each will look white. A white rod in red light will look red, and its shadow will look black. A white rod in green light will look green, and its shadow will look black. A white rod in blue light will look blue, and its shadow will look black. A white rod in front of a red light and a blue light will look magenta, and it will have two shadows, one blue, and the other red. A white rod in front of a red and in front of a green light will look yellow, and it will have two shadows, one red, and the other green. A white rod in front of a blue and in front of a green light will look cyan (or aqua), and it will have two shadows, one blue, and the other green. If the rod is in front of a red light, a green light, and a blue light, it will appear white and have three shadows, one magenta, one yellow, and the third one cyan.

Prism Rainbow

Pure white light and sunlight contain all of the colors of the rainbow. Because different colors of light bend differently in different materials, white light can be split up to form a rainbow by prisms, glass balls, and even special plastic sheets. Rainbows in the sky are from sunlight bent through and off of millions and millions of raindrops.

Beyond the Rainbow – Infrared Light

There are many colors we cannot see. Beyond violet we have ultraviolet and beyond red we have infrared. However, most cameras can see quite well into the infrared, and change it to light we can see. A camera image of a rainbow clearly shows light beyond red that we cannot see. It can also see the infrared light signal from a TV remote.

Rainbow Glasses

Glasses made with special plastic sheets can split light up into colors just like a prism or raindrops. Different sources of light are fun to look at through rainbow glasses. A white light bulb gives off all colors, while a white florescent bulb gives off only five distinct colors.

Atomic Spectra

Light given off or absorbed (soaked up) by a gas is unique, like a fingerprint. We can tell what elements and chemicals are in a gas by the colors of light (called a “spectra”) it gives off. Pure elements give off very clear patterns of colors. We use this to determine what elements and chemicals are in far off stars, in the atmosphere of distant planets, and in giant gas clouds in outer space.

Laser Illusion

In order for us to see an object, light must come from it to our eyes. Mostly we see light from the sun or other sources that bounces off of objects. Occasionally, objects emit their own light. Light travels in straight lines. A laser produces light, but unless the laser light bounces off of something, or shines directly into your eyes (a very bad idea, since this can damage your eyes), you cannot see it. The beam is invisible in normal air. However, you can see the beam if there is something in the air, like smoke, that the light can bounce off of.

Laser Light Show

Very few things are cooler than lots of lasers shining through fog and mist to make pretty patterns, especially if lots of students can help out!

Chinese scientists unveil high-powered laser that can ‘write’ in the air

Chinese scientists have shown off a laser that can create ghostly Chinese characters out of thin air.
Lasers have already been used to create a range of optical illusions, but previously they needed dust or clouds as a medium.

But the researchers behind the new device say it can draw patterns anywhere by using ultra-short laser pulses to strip the electrons off air molecules and turn them into light, creating a ghostly image floating in mid air.

In one demonstration earlier this week at the Hongtuo Joint Laboratory of Ultra-Fast Laser in Wuhan’s optics valley, researchers created characters that were visible from any angle and which researchers could “touch” with their hands.

“With the brand new device, we can draw in the air without using paper and ink”, Cao Xiangdong, lead scientist at the laboratory, told Science and Technology Daily in an interview published on Tuesday.

The device works by focusing high-intensity laser pulses in the air to create plasma, or ionised gas, which emits energy in light form.

The team said they had used a 3D scanner to arrange the pixel dots and form the characters precisely in the air, but did not explain how this works.