Cindy, a High School cheerleader, told her fellow cheerleaders that she was worried about changing for practice in their locker room. It seems that she overheard the Science teacher, Dr. Smith, telling the Principal that he had just invented an invisibility potion which turns a person, who drinks a sip of the potion, completely invisible for 24 hours. The bad news was that someone had stolen a bottle of it from Dr. Smith’s lab. Cindy was worried that the thief was a guy and that he might already be invisible, hiding in their locker room and waiting to watch them undress. Jane, the Head Cheerleader and President of the Science Club, believed Cindy since she knew Dr. Smith was working on the potion, but told her they had nothing to worry about. Since Jane did not steal the potion or know who had stolen the potion, how could she be so sure that their privacy would not be compromised?
Answer
Jane knew that if the potion really did work and could make the thief invisible, it would also make him blind while he was invisible. In order to see light needs to be absorbed by our retinas. If a person were completely invisible all light would pass right though him and he would see nothing. If the retinas were somehow able to absorb light then there would be two black spots in the air where they were and they would not be completely invisible. In this case, light would then hit the retinas from all angles, since the head would no longer be opaque, and blind the invisible thief. Plus the lenses in the thief’s eyes would not work, so no image could be focused onto his retinas. Ironically, if the thief drank the potion he would never be able to see that he was invisible and unless someone told him he would not even know he was invisible. He would only know that he was blind.
