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Homework Help: Snell's Law lake light problem

  1. Mar 25, 2007 #1
    1. Depth of a lake is 1637m. If a beam of light with an angle of incidence of 60.0º enters the water from the air, what is the horizontal distance between the point where the light enters the water and the point where it strikes the lake's bottom?

    2. Snell's Law: ni(sin Өi) = nr(sin Өr)

    3. I'm really lost on this one. I know the angle of incidence and index of refracion is 1(?) How do I go about getting horizontal distance. Is there some equation that I'm not considering? Any help would be appreciated.
  2. jcsd
  3. Mar 25, 2007 #2
    Can you calculate the angle of refraction?
  4. Mar 25, 2007 #3


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    you need to work out [tex]\theta_r[/tex] from the [tex]n_i, n_r ,\theta_i[/tex], then use trig to work out horizontal displacement
  5. Mar 25, 2007 #4

    Yeah, it's 40.5º.OK that helps. But I forgot the trig part of it. Can someone jog my memory? Is it tan(40.5 x 1637)
    Last edited: Mar 25, 2007
  6. Mar 25, 2007 #5
    You can construct a triangle with the angle of refraction, the height of the lake and the horizontal distance now, can't you?
  7. Mar 25, 2007 #6
    Yeah man I can. So, I'm getting 1.63 m for an answer. Right...wrong?
  8. Mar 25, 2007 #7
    Hummm wrong. Look. The refraction angle is measured from the normal line. The normal goes from the surface of lake to the bottom. The length is 1637. Now look at this triangle


    See the right side? This is the height. The bottom side is the horizontal distance and the angle between the left and the right side is the refraction angle. Now you use trigonometry to get the horizontal distance.
  9. Mar 26, 2007 #8
    tan 40.5 = x/depth of lake ( perpendicular by base)

    now calculate x by putting value of tan40.5
  10. Mar 26, 2007 #9
    Yeah that's what I did. I'm probably just screwing up in the math somewhere. So the angle of refraction is 40.5. The horizontal is opposite to it and the adjacent is 1637 m. So it'd be tan (40.5) = x / 1637. Now don't I just solve for x?
  11. Mar 26, 2007 #10
    Answer = 1398.13 m

    They are askin for distance between point of incidence and point of emergence (in ur case its bottom of the lake) . just draw a perpendicular at the incidence wrt the interface
  12. Mar 26, 2007 #11
    oo damn I got that. Thought it was wrong. Alrite thx bro. Appreciate it.
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