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Homework Help: Getting Crabby at Semester's End

  1. May 1, 2004 #1
    We are doing a chapter on color.

    Q. A red crab very deep in water, where sunlight is dim, appears:

    I have seen crabs, reddish crabs, in shallow water, and they looked tan or brown. But in deep water, I am unsure they would be brown. I know this has something to do with the color red getting absorbed as you travel down in seawater. However, the book is saying that the real color of seawater is greenish-blue, and I feel it is more a tea color, more greenish tan. So, I don't know.

    Q. A blue crab will appear black when illluminated with:
    Blue light?
    Yellow light?
    Cyan light?
    None of these?

    I think it's yellow...that's the complementary color of blue...but then again, doesn't that mean it would be white? So I would think it would be RED, perhaps, but that isn't a choice offered.

    Thanks for any help.
  2. jcsd
  3. May 1, 2004 #2
    Im not sure if this would be of any help but i went to lecture last week about using lasers to communicate with submeries. The prof. giving the lecture work for the navy during the cold war, and work not this lasers. He said that the only color laster he could use to communate with subs was blue. Thus a red crab (absorbs every wave length but red) should appear black/dark because their is not red light at the bottum of the sea. This is just an guess, but i hope it helps.
  4. May 2, 2004 #3


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    Science Advisor

    The crab is blue because it only reflects blue light. Which of those is not blue? Blue is blue. Yellow does not have blue in it. "light" probably means white light, which includes blue. Cyan is blue. I think the answer is yellow.

    You can go into MS Paint and check which colors have which components in them.
  5. May 2, 2004 #4

    Doc Al

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    Staff: Mentor

    Yes, the seawater absorbs the color red. As you go deeper, the light will have less and less red. So the "red" crab--and the blood from that shark bite!--will appear black.
  6. May 2, 2004 #5
    Thx to all who answered, I appreciate it. I have one more "crab" question I'll post separately. What is it with these crabs?
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