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Snells Law - Problem understanding question

  1. Sep 13, 2012 #1
    I have no idea what angle it's talking about and also what angle im supposed to get in this problem. It seems to me like the problem is written wrong. Could anyone please let me know if it is wrong or right. If it's a valid problem could you also post a simple MSPaint drawing of how this should be drawn with the 30degree angle along with the angle im supposed to find :) thanks!!

    One of the contests at the school carnival is to throw a spear at an underwater target lying flat on the bottom of a pool. The water is 1.20m deep. You're standing on a small stool that places your eyes 3.10m above the bottom of the pool. As you look at the target, your gaze is 30degrees below horizontal. At what angle below horizontal should you throw the spear in order to hit the target?
  2. jcsd
  3. Sep 13, 2012 #2
    were you given the refractive index of the water in the pool or were you asked to find it.
  4. Sep 13, 2012 #3


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    The problem seems well-posed to me.

    Water has a known refractive index.

    If you know the angle of incidence, you can calculate the angle to which the line of sight is refracted below the water. That's Snell's Law.

    We are told that the line of sight above the water is 30 degrees from the horizontal. That means that it is 60 degrees from the vertical.

    Given the refracted angle and the known depth of the pool you can calculate the point of intersection of the line of sight with the bottom of the pool.

    Then it's just a matter of aiming your spear at that point.
  5. Sep 13, 2012 #4


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    Science Advisor

    Draw horizontal line from your eyes. Your gaze is 30 degrees below that. By the 'alternate angles theorem' for parallel lines, your line of sight also makes a 30 degree angle with the surface of the water.
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