Close ShaveScience brain teasers require understanding of the physical or biological world and the laws that govern it.
Homer had suffered a bizarre accident that affected his eyesight. The doctor said it would be temporary, but for the next 4 weeks, he had to adjust how he did some things. The accident had affected his focal length. He was only able to focus on objects that were 6 or more feet away from him, anything closer than 6 feet was just a blur.
Homer was used to shaving up close in front of his bathroom mirror. Now after the accident, how close could Homer get to the mirror to see his face clearly enough to shave?
Your focal length in a mirror is your distance away from the mirror, plus the distance of the object away from the mirror. When you look at an object in a mirror, you are seeing a virtual image the exact distance it is away from the mirror. So, if you are standing 3 feet away from a mirror, and focus on your face, you are actually focusing at 6 feet.
To try this out, put a piece of tape on a mirror, about eye level. Standing in front of the mirror, focus your eyes on the tape, now your face is out of focus. Focus now on your face, and the tape is out of focus.
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