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Homework Help: Optics question

  1. Jul 17, 2006 #1
    Direct sunlight hits the water at an angle 30 degrees to the air-water surface normal. At what angle does this light penetrate into the water? If this light is reflected off an underwater object whose normal is at 10 degrees to the vertical then in wat direction does light travel through air after leaving the water?

    using some scrap papers I manage to find the answer for the first part to be 22 degrees. the seconds part of the question i do not understand what "normal is at 10 degrees to the vertical" means?
  2. jcsd
  3. Jul 17, 2006 #2


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    If the normal to the reflecting surface is is making 10 deg with vertical, so what is angle of incidence at that surface? The ray will be reflected according to laws of reflection, and then again refraction is to be considered. (You have to consider two cases).
  4. Jul 17, 2006 #3


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    Is there more information for the second part?

    The underwater object - let's assume a flat plate (surface) - has a 'normal' associated with the surface area. The 'normal' is a unit vector which is perpendicular to the two vectors (dimensions) which could be used to describe the area of a surface.

    So the normal of the surface of the object is tilted 10° from the vertical, and the vertical is parallel to the normal of the water.

    But the question I see is - is the normal of the underwater object tilted toward the incident light or away. That would make a big difference in the angle between the incident light and the object's normal.
  5. Jul 17, 2006 #4
    nope i have typed out the question exactly as it is in the textbook, its more of the language and the wording of the question that i dont understand than the concept.
  6. Jul 17, 2006 #5


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    Is the concept of the 'normal' understood now?

    Refraction, which is the bending of light when passing across a material interface, is described by Snell's law.

    This is useful.

    http://scienceworld.wolfram.com/physics/SnellsLaw.html - shows illustration of normal, which is mentioned in discussion.


    Also - http://www.glenbrook.k12.il.us/gbssci/phys/class/refrn/u14l2b.html [Broken]
    The latter has some reasonable illustrations.

    The for 'reflected' light, the angle of reflection = angle of incidence.
    Last edited by a moderator: May 2, 2017
  7. Jul 17, 2006 #6
    Yes I understand the concept behind it. Thanks for the links but it is the question I do not understand. The 2nd part of the question has a sentence "normal is at 10 degrees to the vertical". The normal is always perpendicular to the surface, so what does vertical refers to?

    Thanks :smile:
    Last edited by a moderator: May 2, 2017
  8. Jul 17, 2006 #7

    Doc Al

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    the surface is tilted

    "vertical" means what it usually means: up and down. Yes, the normal is perpendicular to the surface, but--in this case--it's not vertical. That means that the reflecting surface is not level with the horizontal--it's tilted 10 degrees. :smile:
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