Why does your hand look dry underwater?

by Stephanie Chasteen on January 25, 2010

Stick your hand in water and pull it out. You can tell that it’s wet, it “looks” wet.

But then try this. Stick your hand underwater and look at it while it’s still underwater.   It doesn’t really look wet.

And even more striking — Look at your wet hand in a mirror.  Now plunge your hand underwater, and look at it in the mirror underwater.  It not only doesn’t look wet, it looks bone dry!

It’s a pretty simple answer to a neat little experiment.  But before I give you the answer, think a moment.  How might you try to figure out the answer?  What are some tools you could use to figure out why this is the way it is?

Well, when your hand is wet, your eye can tell that it’s wet because there’s a layer of water on the hand.  Light reflects from that water more strongly than it does from your skin, so it looks “shiny” because of that extra reflectivity.

When your hand is entirely under water, though, there is no surface layer of water on the hand.  The entire hand is under water, so the only shiny reflection is from the surface of the water itself.

But what about when the extra trick with the mirror is added?  All I can think is that this tricks the mind a little bit.  Instead of looking at the hand underwater, you see a reflection of the hand, and so it’s out of context.  Your mind just sees the hand, and doesn’t compensate for the fact that it’s underwater.  I’d love to hear a more rigorous explanation of this though!

{ 4 comments }

sciencegeekgirl January 26, 2010 at 3:08 am

Paul Doherty (whose Q&A with a teacher sparked this post) wrote me on Facebook:

“Sounds good to me. Now catch a bubble under the palm of your hand and let it roll around, it looks like a blob of molten metal in your palm.”

James January 26, 2010 at 11:37 pm

Hi ScienceGeekGirl!
I was wondering about the effect of the mirrorand how it seems to amplify the dry appearance of the hand. I would speculate that less light scatter off the surface of the water is reflected out of the mirror. This would further the effect of the mind’s interpretation of the hand.
I haven’t had a chance to try it out myself but are the effects lost if the water is in motion?
JB

sciencegeekgirl January 31, 2010 at 1:24 am

Yeah, the effects are mostly lost when water is in motion, but that’s just because you can’t see through the surface of the water when it’s moving.

Give it a try, it’s pretty interesting!

sciencegeekgirl February 25, 2010 at 3:49 pm

A comment from the Facebook version of my blog:
“One of (the many) fun tricks we did in college was to put on a scuba mask and then float upside down in a swimming pool, just below the surface, at night. Then watch the under surface of the water and your feet as you stick them up and out of the pool into the air. I can’t remember if they looked wet or dry, but I do recall it was a strange sight.”

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