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Refraction - what's happening for real

  1. Oct 2, 2008 #1
    Ok. I don't understand this refraction stuff. The books all show the light bends down when light from the air enters water ah, change in medium. But when I clean our pool the rod appears to bend up. My Dad is a jock and he says to hunt for fish you throw the spear in front of the fish you see in the water. So what gives here. Doesn't make sense to me that the pole appears to bend up when the books say the light bends down!!!!!!!!!!!!!! Confused.
  2. jcsd
  3. Oct 2, 2008 #2


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    Actually those two things are completely consistant. It's best understood by drawing a simple ray diagram. Imagine the path that a ray of light scattered from the tip of the pole will reach your eye. As the ray exits the water it will bend away from the normal and travel toward your eye at a "lower" angle as compared with the angle that it would approach your eye if the light travelled in a straight line. This fools your brain into thinking that the tip of the pole is further forward and at a shallower depth than it really is (hence the pole appears to bend upwards.)

    BTW. You'll probably need to try and draw the ray diagram for yourself to really understand what I'm saying. I dont have time to post an image right now.
  4. Oct 2, 2008 #3


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    The appearance of the rod is due to light from the pole in the water to the air and finally to your eye, which is the opposite direction of travel from what you describe in your second sentence.

    Both are described by Snell's law, as uart says.
  5. Oct 2, 2008 #4
    Thanks, I just found a drawing on Wilki, which I'm looking at, I think its what you are saying. Still seems confusing, though. It seems to me the pole appears to be bending up because the light is bedning up rather than down as the books seem to show. Am I a dumb nerd or what? LOL
  6. Oct 2, 2008 #5
    Thanks, I'll look at Snell's law too.
  7. Oct 2, 2008 #6


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    No it looks like this. The light bends "downward" as you described, but the apparent position of the tip of the pole appears to be higher than it's actual position making the pole appear to bend upward. The reason for this "apparent" posiiton is that your brain assumes that the light travels in a straight line and therefore assumes that the tip of the pole lies in the direction that the light from that tip is entering the eye. The solid lines shows the actual pole and light ray while the dotted lines show the apparent pole position and the apparent light ray (the one that you brain thinks it should be becuse it assumes a straight path).

    http://img220.imageshack.us/img220/3506/refractioniy9.png [Broken]
    Last edited by a moderator: May 3, 2017
  8. Oct 2, 2008 #7
    uart Thanks, I'm starting to get it. Guess my brain works backwards. LOL That bit on Wilki on Snell's law with the expanding rings put me in a trance but it finally started to make sense too. This concept is a hard one for me to get my hands around, right now I just wanna choke it.
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