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MIT student blogger Laura N. '09

Ponderings of a lifeguard by Laura N. '09

Why are white water rapids white?

So the other day at work, I was sitting at the bottom of one of the water slides. And I was bored out of my mind. There were about 3 people in the entire park, and my entire job was to make sure that none of them…I don’t know, died…when they got off this particular slide. So I was just sitting there, trying to ignore the incessant Beach Boys music they play on loop the entire @#^?*&! summer, watching the water cascade down the last 3 feet of the slide and crash into a continuous white spray at the bottom.

And suddenly I wondered, why is it white?

So I sat back in puzzlement and continued to stare at the water, thinking. I supposed that since the water was moving quickly, so this somehow scattered the light or kept it from being absorbed or from shining right through. But given the speed of the water coming down a slide and the speed of light, this seemed a little hokey. Besides, after crashing into the bottom, the water sprayed across the small pool and remained in its foamy white form as it drifted lazily towards the drain. Of course, the foam is really just a lot of little bubbles gathered together, but still… if there’s nothing in the water that is white, how does it suddenly turn white when it gets churned around a lot? It’s a physical change, not a chemical change, so what gives?

This has now been bothering me for over a week. Whenever I’m at a spot with any kind of cascading water (which is about half of them) on a slow day, I find myself puzzling the many intricacies of turbulent flow, how it disrupts normal optics, and a million other little questions. But the big one, mainly, “Why are white water rapids white?” is really driving me nuts.

If there’s anything I’ve learned by being a blogger this year, it’s that most of the incoming freshman are smarter than me. So I’m sure some of you know the answer to this. Come on, help me out.

In other news, I finally figured out how I’ll be able to update my blog this summer! I feel so guilty for my prolonged silences, but unlike some of my counterparts who are traveling around Europe and doing other cool things, I spend every day staring a pool after pool of water, basically waiting for some major accident to happen. I don’t have much to talk about.

So aside from answering all of the questions that you guys aren’t asking me (*glare*), I’ve decided to review books.

See, reading is a huge part of my life. My mom used to beg me to put the book down and go outside. (My childhood is full of my mom begging/forcing me to do things that most parents beg/force their kids not to do. There are some funny stories in there.) One of the 3 million reasons I love the summer is because I finally have time off to just read. My mom is convinced that there’s a cute guy who works at the library, because she can’t figure out why else I would want to spend so much time there. Anyway, rest assured that I will spend the entire summer reading anything and everything I can get my hands on. I’ve been known to walk down an aisle of the library and pick a random book off the shelf to read. Since this activity constitutes the most interesting thing I’ll be doing all summer, I will blog about it. I’m also open to reccomended reading (titles, topics, authors), so comment away.

I know this is a lame way to blog all summer. So ask more questions. =P

Also, while reading Mollie’s blog, I realized that today was my blog anniversary. My unoriginally named blog is exactly 1 year old. How cool is that?

10 responses to “Ponderings of a lifeguard”

  1. Mollie says:

    I’m sure at least some of the foam is due to proteins and oils sloughed off the skin of the people on the slides — when you shake up a water/protein solution (for example, by sending water + people goo down a water slide at high speed), you get bubbles.


  2. Mom says:

    I’m an MIT mom, and the mom of an MIT hopeful, so I read these blogs. I’m a reader too. In junior high and high school I used to collect the biggest books I could find in the library, then let the librarian cull through them for books she thought I’d like. I found some great books that way. I like your idea too.

    Happy Anniversary!

  3. Dan says:

    I believe that the white color is caused by the water mixing with air bubbles as it goes through the rapids.

  4. Anonymous says:

    Happy anniversary!

    Your blog’s really cool you must be cool.

  5. Anonymous says:

    I just noticed… without the spaces i.e. the ‘iff’ sign can’t be typed!

    I had meant this

    Your blog’s really cool you must be cool.

  6. cristen '10 says:

    Ok, you want more questions. I just thought of a random one. I want to learn how to play the violin. Can I walk into MIT, grab a random violin, and play with it?

    No, I’m serious.

  7. Anonymous says:

    Maybe for the same reason clouds are white.

  8. SUchi says:

    My mom used to beg me to put the book down and go outside. (My childhood is full of my mom begging/forcing me to do things that most parents beg/force their kids not to do. There are some funny stories in there.)

    – My mom’s the same way. She never got why books fascinate me. I luv to read!!!!!!!!! nd I wouldn’t mind some reviews on good books.

  9. Shannon says:

    Yeah, I agree with you Dan, but then how come the addition of air (which is, let’s assume, perfectly clear), make white?

    Well, how many chemicals are in that pool?

    Wait, how about this… since water generally reflects color (blue on sunny days, grey on cloudy ones), and the bubbles are thin layers of water on almost every side, maybe it’s just the overload of colors that it’s exposed to on all those sides. White is all the colors combined, right? I just remember it as the opposite of paint.

    Just a thought, thoroughly non-physics.

  10. guess who says:

    cavitation. the currents flowing in the water can lower the pressure at discrete points enough for tiny amounts of water at these points to “boil”, releasing little bits of gas into it. on a macro scale, that is.

    plus all the water crashing down brings air and bubbles with it.