The complex nature of invisibility
You can both see and not see things
Invisibility means something is, well, not visible. While invisibility is often associated with science fiction, fantasy, and horror, objects are invisible under normal, mundane circumstances.
Objects can be hidden from view because they are behind other objects, too small or too far away to see, or obscured by dark or fog.
Speaking more science fictiony, there are different theoretical and in practice ways to make objects invisible. They use physics, human physiology, and psychology.
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Transparency
One physical way, and the way most people think of as invisible, is to make something transparent. Transparent means light passes directly through the object. There are natural and human-made transparent things. Transparent objects includes glass, plastics, water, cellophane, and jellyfish. Almost nothing is 100% transparent though. If you walk into a glass door, it’s usually because you aren’t paying close attention.
If you were completely transparent you’d be blind because to see the eyes must absorb (retain) light (the opposite of transparent). Note that vampires in movies are sometimes shown as invisible except for their eyes.
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Bending light
Another way to make an object invisible is to bend light around it.
Mirages give an idea of how this can be done. In the heat of the desert, light can be bent unusually to make the sky appear to be a pool of water in the desert or a road. The mirage doesn’t involve the item being hidden, but ‘moved’ to an unexpected place. Funhouse mirrors also show how light can be bent to distort and hide objects from view. Looking at yourself in a funhouse mirror, your head can suddenly disappear.
There are actual but very early stages ways to make objects invisible by bending light around the objects. Manufactured materials can bend light to make an object disappear from view. These technologies are being developed by the military and university labs.
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Obstructing the views
Many items are made invisible by obstructing the viewer’s view. Turning off the lights or standing in front of an object are two examples. Bright glare in the viewer’s eyes obscures the view. Warplanes electronically ‘jam’ radars. During World War II, airplanes dropped small metal ribbons to disrupt radar.
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Invisibility usually happens only at one level
When something is made invisible, it is usually invisible at only one frequency of light. It can be seen at other frequencies.
A soldier hidden by the shroud of the night can be seen with night goggles that detect infrared light. The bones inside our bodies are hidden from normal view but are clearly seen under X-rays. A jet fighter can make it invisible to radar but can be seen with the naked eyes. In horror movies, the invisible man can often be detected via sound, smell, the appearance of footsteps of things moving around.
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Invisibility due to human psychology and physiology
Objects in plain sight can go consciously unseen by humans. This can be due to the human optic system and due to human psychology.
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Blind spots
All humans have blind spots, which are spots where the eye cannot see. The blind spot in an eye corresponds to the spot on the retina where the optical nerve connects the retina to the brain. At this spot there are no light-detecting cells and, thus, this spot cannot detect light. A small object can disappear from view at the spot.
In everyday life, the blind spot goes unnoticed. This is in part because the eye is constantly looking around, getting a wide and varied range of views. It is also in part because the brain uses the information from both eyes to create a single mental vision. What one eye misses, the other often picks up.
As its optical nerve connects differently, the octopus has no blind spot.
Detecting your blind spot
To detect your blind spot using the above red dot/green dot picture on the next page, close your right eye and look at the GREEN dot. Slowly move your head toward the picture. At one point the RED dot will disappear. Notice that the missing spot is filled in white by your mind, so it appears as if nothing is missing from your view. This illustrates how your blind spot goes unnoticed during daily living. Many people live their entire life not knowing they have a blind spot.
Color blindness tests show that objects of certain colors can hide from view from those with limited color vision. People with poor color vision cannot make out the letter or number symbols that those with good color vision can see. This is a form of camouflage.
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Camouflage
Camouflage involves making things that are visible hard to distinguish from its surroundings. The viewer can see the object– it registers and is often in focus in the eyes and mind–, but does notice it consciously. The camouflaged object hides in plain view.
A new book will be noticed if placed in the middle of the floor or the kitchen table, but will go unnoticed if slipped amongst other books on a book shelf. The viewer sees the book– it registers in the eyes and brain–, but the viewer doesn’t consciously notice it.
With a chameleon standing in front of a rock of the same color, you can see all the same details as when the chameleon is standing in front of a white sheet. It’s just that against the rock, you lose contrast and cannot consciously identify it.
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Psychologically invisible
Many things are overlooked because they don’t meet expectations. As with camouflage, things can be hidden in plain view.
A human does not and cannot simultaneously focus on all information in a scene. Humans don’t have the mental capacity. Humans focus on some things and ignore others.
When you enter a room, your eyes are drawn to something or things. Perhaps you focus on the gracious hosts, perhaps a statue to the side. If there is a rat in the middle of the floor, your immediate perception will be of the rat and not of the rose wallpaper.
If you enter the room and there is an attractive nude, you likely won’t notice what is on the coffee table. You might not even notice the coffee table. After blushingly excusing yourself and scooting out of the room, you may not recall the existence of a coffee table, but it was there right in front of your eyes.
Read the above text. Then look at it again. Did you skip over the second ‘the’?
Very helpful, thanks.