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Reflection/Refraction water tank question

  1. Jul 19, 2015 #1
    1. The problem statement, all variables and given/known data

    The 80-cm-tall, 65-cm-wide tank shown in the figure is completely filled with water. The tank has marks every 10 cm along one wall, and the 0 cm mark is barely submerged. As you stand beside the opposite wall, your eye is level with the top of the water.

    Part A asked "Can you see the marks from the top of the tank (the 0 cm mark) going down, or from the bottom of the tank (the 80 cm mark) coming up?" I already answered this question: We can see the marks from the bottom of the tank coming up.

    Which is the lowest or highest mark, depending on your answer to part a, that you can see?

    I have attached an image I found online of the figure

    2. Relevant equations

    The only equation that I can think of would be Snell's Law n1sinθ1=n2sinθ2

    3. The attempt at a solution

    I am not sure how or where to start. If the 0 cm is at eye level, then wouldn't that be the highest mark you can see? Should I be sketching rays from each mark to the eye? If so, wouldn't they all reach the eye?

  2. jcsd
  3. Jul 19, 2015 #2
    Your idea to sketch rays from each mark to the eye is great! Just remember that your eye isn't in the water; your eye is in the air. If you'd like, you can imagine that your eye is very close to but not touching the edge (e.g. 1 mm or 1 cm). The wording in these sorts of problems can be a bit confusing at first.

    Next time you're swimming in a pool, see if you can test this out.
  4. Jul 19, 2015 #3

    Thank you for replying! So as the rays head toward the eye, does the incident ray hit the edge of the tank (air-water boundary?) and then refract in air before it hits the eye? Sorry, I'm still a bit lost. Wouldn't they all still hit the eye?
  5. Jul 20, 2015 #4
    Yes, there is refraction at the air-water boundary on the horizontal surface of the water. The light then has to travel through some air -- even if it's less than 1 mm -- before it hits the eye.

    Try it out with some rays, maybe starting 10 cm below the surface, and you'll see whether all of the rays hit the eye. You're correct to use Snell's law for this.
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