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Homework Help: One More Crab Question

  1. May 2, 2004 #1
    You'd think I could figure this out based on the answers to the previous crab-color questions, but I can't.

    Q. Nellie Newton wishes to "tag" a blue crab with a red laser pointer...She is standing on a pier. The crab is down in the seawater. Should she aim the pointer ABOVE where she thinks the crab is, DIRECTLY AT the crab, or BELOW where she sees it?

    I was thinking ABOVE where she thinks she sees it, because usually, if you were going to spear it, with an actual spear, you'd aim BELOW it. But with the red light being so weak in the seawater, maybe you have to aim it ABOVE in order to get any to the crab. Plus, the crab being blue, doesn't that mean it is bending the light away from her as it comes out the water? And the red bends DOWN?

    Any help appreciated. I just got back from the E.R. with my poor eyes, I can't actually do this experimentally, they are practically swollen shut (see what studying physics does to a person?). Hope I am typing this correctly.
  2. jcsd
  3. May 2, 2004 #2

    Doc Al

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    dispersion due to color

    No, this one's different. It has to do with refraction of light and how that depends on color.

    When light is refracted by glass (or water) which color gets bent the most, red or blue? Think of a prism. It just so happens that the amount of bending depends on the color. The index of refraction is a little bit greater for smaller wavelengths (like blue) compared to longer wavelengths (like red), so blue light bends more and red light bends less.

    If she shined a blue laser at the crab, aiming straight for where she sees it, would the laser "tag" the crab? Yes. You know that it will because it's the blue light that allows you to see the crab; so whether it's going from crab to you, or you to the crab, the light follows the same path.

    Now, of course, the blue light--when it hits the water--bends downward, right? (Please draw yourself a picture of the light rays bending.) So... what if she shined a red laser "straight" at the crab? Red light also bends down when it hits the water. But does it bend more or less than the blue light? From what I said above, you know that red light bends less.

    I leave the final step to you. Does the red light have to be aimed a little higher or a little lower that the apparent position of the "blue" crab?
  4. May 2, 2004 #3
    Thank you, Doc Al.

    I'm thinking Nellie ought to leave the crab alone.

    I think she ought to shine the laser a bit below where she is seeing the crab. Otherwise, she will overshoot the crab. ?

    But I guess she'll never really know if she "got" it or not, since the red light will be absorbed in the seawater and it will just look like a black dot on the crab, and who can see that, if her eyes are also swollen almost shut?

    I tried to draw the pictures like you said. I have a large assortment of crabs in different positions, and Nellie on the pier, and lines drawn from her eyes to the crab's eyes, their eyes to her laser-pointer, and just to make things even, I gave a few crabs laser-pointers and have them aiming at her. It's a mess. If I make it out of this class, it will be a miracle.
  5. May 2, 2004 #4
    No, no, above the crab! She should aim above!!! I think!
  6. May 2, 2004 #5

    Doc Al

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    Interesting point. If the water's not too deep, the seawater won't absorb that much of the red light. But, since the crab is "blue", the crab will absorb the light and will not reflect it back. I guess Nellie will never really know. (Unless the crab jumps out and bites her for shining a laser in its eye.)
  7. May 2, 2004 #6

    Doc Al

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    Bah! Draw that picture! And reread what I wrote.

    Don't guess... give me a reason.
  8. May 2, 2004 #7
    Well, I am trying!!!


    I am drawing the picture two ways. In one way, the lines from the pointer go into the water like "L's," and the other way has them going into the water like backwards hockey sticks. With the L's, I have to aim below. With the hockey sticks, I have to aim above. See the problem?
  9. May 3, 2004 #8

    Doc Al

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    Here's a pic for you


    I drew my version of what's going on. I'll try to attach the file. In my drawing, the red circles represent Nellie's bloodshot eyes and the blue squares represent the crab.

    Let me know if it makes sense.

    Attached Files:

  10. May 3, 2004 #9
    Doc Al, you're so nice!

    Thank you for the drawing. So, I can now see she has to aim below the Apparent Crab in order to get it tagged. The line bends like a backwards hockey stick. Maybe the meeting of the surface of the air and the seawater is sort of like a mirror that lets the light in the back, through the coating, so to speak, and then bounces it off at that funny angle as if it came from under the water.

    Many, many thanks; you are a lifesaver. We have four chapters in 4 lectures and I just could not get the whole Light business. I kiss your feet.
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