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B Problem with light grazing along the liquid surface

  1. May 10, 2017 #1
    Let's suppose we have a cuboidal tank filled with liquid of density rho,such that
    Critical angle of medium is (Ic) now it's
    Clear that light rays from the medium that strike at proper critical angle will
    Graze along the surface
    Now if someone would be along the path of the rays receiving them into his eyes I'm not able to visualize what he will see as light from many parts will be coming into his eyes at once .does that means he will be seeing many parts of tank together at same place this is confusing .please let me have deeper insight into situation

    Thanx to all those who will help
  2. jcsd
  3. May 10, 2017 #2


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    You have it right, I think. You won't see an image of the bottom of the tank, just the sum of all the light coming from the bottom (and sides) of the tank. You could imagine it as an image squashed down to zero height.

    You could try it. Get a shallow box or a dish with a pattern on the bottom. Fill it to the brim (can you get an inverted meniscus?) with water. Put it near the edge of a table, let it settle, and then look at it from above. Then just kneel down until you're looking along the surface. As you do, you'll see the pattern on the bottom squish down to nothing.

    Compare what you see when the dish is empty.
  4. May 10, 2017 #3
    Also i had a problem on applying reversibility principle of light in this case
    Like how light rays from different parts of tank are grazing along surface ,the principle of reversibility predicts that light from eye that grazes along surface should also enter the medium of liquid at all points at an angle equal to critical angle of medium
    But this seems weird pls correct me if I'm wrong somewhere
  5. May 11, 2017 #4


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    Why is the sky blue? Because light both passes through it and scatters off it. That's really only a plausibility argument for the idea of light scattering in multiple places along the surface, butvthe idea isn't totally absurd.

    In fact, ray optics is an idealisation. If you think about what the water-air boundary really is - a collection of molecules jiggling and bouncing around - do you really think light will travel neatly along the continually shifting surface? You're going to have to go far beyond ray optics to get a completely satisfactory description of it.
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