July 28, 2004
What would it be like if you could become invisible? What would you do? How would your friends react? What would happen to you?
Becoming invisible has been a part of all sorts of stories, movies, and TV shows. In Harry Potter and the Sorcerer's Stone, Harry gets a shining, silvery invisibility cloak to wear whenever he needs to sneak somewhere without anyone seeing him. In the 1992 movie Memoirs of an Invisible Man, the character played by Chevy Chase becomes invisible as the result of a lab accident. A few years ago, the Sci Fi Channel featured a series called The Invisible Man. There are lots of other examples.
In many cases, these stories say very little about how something actually becomes invisible. They leave it to magic, a potent potion, mysterious rays, an explosion, or another handy device. A few stories, however, go to some trouble to explain scientifically how an invisible being might be possible.
When H.G. Wells wrote his book The Invisible Man in 1897, he included a lesson in physics to show how a person could make himself invisible. The key idea is that an object is visible only if it affects light in some wayby reflecting, bending, or absorbing it.
For example, you see a red box because it reflects red light and absorbs the rest of the colors that make up white light. A colorless, transparent glass marble is less visible, made evident only by the way it bends and reflects light. If you were to drop the marble into water, the marble would become even less visible because light passing from water to glass bends less than light passing from air to glass. That's why transparent, colorless jellyfish are practically invisible in water.
The degree to which light bends, or refracts, upon entering a material at some angle is known as the material's index of refraction.
In his book, Wells explained that the tissues that make up the human body are already remarkably transparent and colorless. That's why it's often hard to see tissue samples under a microscope, unless the samples are stained. If you got rid of the red color of blood and any coloring in skin and hair, then lowered the body's index of refraction to that of air, you would become virtually invisible.
That's what the physicist who becomes an "invisible man" in Wells's book proceeded to do. "My limbs became glassy, the bones and arteries faded, vanished, and the little white nerves went last," Wells wrote. "At last only the dead tips of the finger-nails remained, pallid and white, and the brown stain of some acid upon my fingers."
An eye has a lens (arrow) that focuses light onto a retina at the back of the eye.
There's a catch, however. An invisible man would be blind.
In order for someone to see, light has to pass into an eye, where a lens bends the light and focuses it onto the eye's retina. But if the lens has the same index of refraction as the rest of the body, and the body has the same index of refraction as that of air, the lens wouldn't bend light at all. So there'd be nothing to focus the image.
Moreover, light must be absorbed to some extent by sensors in the retina so that the sensation of sight is produced in the brain. The eye of an invisible man would have to absorb enough light to see but not so much that the subtraction of light would be noticeable.
Interestingly, many sea creatures are colorless and transparent. When they are caught and brought to the surface, the most striking thing about these creatures is often their little black eyes.
So, next time you read about an invisible being or don't "see" one in a movie or TV show, think about eyes and light. Does the writer pretend the problem doesn't exist, or does he or she find a clever way around it? And what about food and digestion? That's another topic that doesn't get the attention it deserves in the world of an invisible man.I. Peterson
The full text of The Invisible Man by H.G. Wells can be found at www.paradoxcafe.com/relax/invisman/invisman.html (Paradox Cafe).
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