It's a good thing this never came up on a test
May 25, 2010 5:27 PM Subscribe
I think I heard a lie from a cave tour guide as a kid, and it's only recently that I realized I took it for granted as a property of optics. Can you confirm this?
posted by Countess Elena to science & nature (19 answers total) 3 users marked this as a favorite
When I was a kid, I got to go on a couple of cave tours, the well-lit, hour-long, perfectly safe kind. I think this particular one was in Arkansas.
There was a wooden footbridge that we crossed over a pool of perfectly still, clear water. The bottom was covered with smooth stones, and it looked about two feet deep. It was lovely; I think I may have stopped to reach into the water. The tour guide asked us to guess how deep we thought the water was. It might look just a few feet deep, he said, but that was only because this water was so perfectly pure and still that it looked clearer than any other water, and appeared very shallow. The water was actually sixty feet deep, he said, so be careful! Naturally I clung to my dad after that.
It only occurred to me the other day that I had still believed him all this time. That's the perfect lie to tell to get a kid to behave and stay on a footbridge! I've done a lot of swimming and scuba diving since, and I just assumed that, since all water is impure in comparison to an underground cave pool, there was never any reason for me to see that illusion again. Now that I consider it, I feel like Karl Pilkington for not having questioned it. My seventh-grade science understanding of refraction isn't helping me, but, like all of us, I do know that a perfectly clear piece of glass can, under the right circumstances, make something look closer.
So, in sum, is it ever true that water with a lack of particulate matter will present the illusion, from above, that the water is much shallower than it is?