# Our Amazing Eyes

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Human eyesight is a wondrous mechanism. On the one hand, we can see an object whose light takes over 2 million years to reach us (M31, the Andromeda galaxy, twin to our own Milky Way). On the other, we can see something a mere molecule thick! Want to try to guess how, before reading on?

In their book and PBS television series The Ring of Truth, Philip and Phylis Morrison asked, "How close to atomic size can we perceive with our unaided senses?" Their imaginative response was to place a quarter teaspoon of olive oil on the surface of a pond. As it spread, the film of oil could easily be seen on the otherwise rippled water. At its limit, it covered an appreciable area, some 2,000 square feet. A simple calculation shows that its thickness -- rather, thinness -- is a mere five-millionth (0.0000002) of an inch. That is, the diameter of a single molecule of oil.

If we were to start off with the depth of that film of oil and multiply it by 10, then multiply that by 10, then that by 10, and so on ... how many times would we have to repeat the process until we arrived at the distance to M31? Let's do it in stages.

Multiply the oil thickness by 10 once, twice, thrice, four times -- and you've got the thickness of a typical human hair. Four more times and you're at the size of a cat (that is, 0.0000002 x 10^8 = 20 inches); another nine times and we've passed the moon. In all, starting with the thickness of our original film of oil, it only takes a total of about 30 multiplications by 10 (10^30) to reach M31.

What an impressive range of sizes -- 30 powers of 10, from the molecular to the galactic -- that we can appreciate with nothing more than a pair of unaided eyes connected to a three-pound brain.

Resource: For a visual display of powers of 10, galaxies to quarks, check out http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/index.html

Barry Evans doesn't advocate pouring oil into Humboldt Bay -- or anywhere. He lives in Old Town Eureka.