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Homework Help: Why does a straw look bent in water?

  1. Jan 12, 2017 #1
    1. The problem statement, all variables and given/known data
    https://www.highlightskids.com/media/kids/highlightskids/images/thumbs/sciQuestions/sq1012_put-a-straw-in-a-glass-of-water_main.jpg [Broken]

    2. Relevant equations

    3. The attempt at a solution

    From my understanding, this has to do with refraction. Here's my answer:

    Light usually travels in a straight line to our eyes but if the light goes through a different medium, it changes direction.

    So as we look at the straw in the glass, the light from the top part of the straw travels straight to our eyes whereas the part of the straw that's underwater has light that is refracted since it goes from air to water, back to air again, so the light travels to the eye at a slightly different angle therefore making the straw look bent.

    The part that's not quite clicking with me is, so what if the angle to our eyes has changed? Why does that mean the straw now looks bent?

    I've been through a few videos but they don't seem to describe why exactly this happens.
    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Jan 12, 2017 #2


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    Staff: Mentor

    The light from the part of the straw that's underwater does not start out in air, but in water. So it's just water-glass-air. Light from the straw that's above the water goes air-glass-air.

    It would be very beneficial to draw a diagram and do a simple raytrace from parts of the straw to your eye. Do you know how to do that?
  4. Jan 12, 2017 #3
    Here's a diagram I've drawn based on what I've read so far.
    WhatsApp Image 2017-01-12 at 16.38.36.jpeg

    So the object is actually at 'object 1' but due to refraction the observer ( and because the brain judges the image location to be where where light rays appear to originate from) deems the object to be at 'object 2'.

    Is that correct?
  5. Jan 14, 2017 #4


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    Science Advisor
    Homework Helper
    Gold Member

    You do not seem to have taken in what Drakkith wrote. Your diagram shows the light going air-glass-air, but, as seen in the picture, the part of the straw above the water only appears slightly displaced.
    Draw a diagram showing what happens for the part of the straw below the water level. Label all the angles to the normal and consider the relationships between them.
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