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B Why does flowing water cast a shadow?

  1. Nov 10, 2018 #1

    Wrichik Basu

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    I found that water flowing from a tap is casting a shadow, somewhat like this:


    If you see through the water flowing from the tap, you can see that the pink colour of the wash basin is visible almost clearly.

    What is the cause of the flowing water casting a shadow? Is it because the turbulence prevents any light from getting refracted through the stream, thereby casting a shadow?

    N.B.: Maybe this question has been asked before. If so, moderators may delete the thread.
  2. jcsd
  3. Nov 10, 2018 #2

    Buzz Bloom

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    Hi Wrichik Basu:

    The movement of the water is irrelevant. Imagine a transparent glass tube. If it is properly made, when it is empty it will not cast a noticeable shadow. When it is full of water, then it will.

    I added "noticeable" after the original post
    Last edited by a moderator: Nov 10, 2018
  4. Nov 10, 2018 #3
    Part is down to refractive index...

    Analogy with a rainbow, where part of the sky just off the bow is slightly darker, some light having been diverted...

    And consider the patterns on the bottom of a swimming pool caused by sun-lit ripples...
  5. Nov 10, 2018 #4


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    A thin, flat sheet of glass would cast only a faint shadow, but a hollow glass tube would cast more of a shadow and a solid cylindrical glass rod would cast even more of a shadow. You might like to think why the shape of the glass makes a difference. And you could try a practical experiment with a drinking glass or a clear glass bottle.
  6. Nov 10, 2018 #5
    If you turn off the room light and shine a light beam (eg. laser pointer) at the water you will see that most of the light is refracted away from the geometrical shadow area.
  7. Nov 10, 2018 #6


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    But this is not an equivalent scenario at all.
    The glass tube is round, not flat, so the tangential edges of the glass tube will be quite visible in the shadow, and
    the glass tube has no "wood pane" to mask the side-by-side difference.

    A glass tube will most definitely cast a shadow.
  8. Nov 10, 2018 #7


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    The simplest answer to the OP's question (as has been stated by others) is that the column of water is a cylindrical prism (with a higher index of refraction than the surrounding air). It refracts the light passing through it. This means the rays from a specific light source will be directed obliquely (to parallel rays), and not fall directly behind the column.

  9. Nov 11, 2018 #8


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    However, when we look at an object through the falling water, it's also necessary to explain why our eye is not equivalent to the ceramic of the sink. The effects require particular circumstances to make them particularly noticeable - water from a tap is a very common example but there are others where it doesn't happen as described.
    The shadow is not particularly deep, in that picture but it is easily discriminated by our eye so it is subjectively significant. The scattered light lands on either side of it, increasing the illumination but that is not perceived because it is diffuse. Objects on either side of the water have images 'in' the stream - you can see the fuzzy images of the actual shadow and the tap on the ceramic, through the water. I think they are, in fact, images very near to the water stream (Virtual or real, depending on the details), which our eyes can perceive but which will not form identifiable 'shadows'.
    There will be instances where the view through a water-filled tube will be definitely darker than the scene behind it. If the tube is in front of a light coloured stripe on a black background, the view through the water will be darker than the stripe because the image contains parts of the black background as well as the stripe.
    Sorry but I don't have time to do a picture. Anyone can verify this with a glass of water if they choose a suitable place in their house. Squinting with the eyes can blur the image and make the effect more obvious.
  10. Nov 11, 2018 #9

    Vanadium 50

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    This thread is (unnecessarily) becoming a bit of a mess, with people editing messages to say different things and the mentors deleting correct explanations.

    Only invisible objects cast no shadows. A column of water is not invisible (for reasons mentioned above by some), therefore it will cast a shadow.

    Be careful! That may cause your message to be removed!
  11. Nov 11, 2018 #10


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    I can't even see it. :biggrin:
  12. Nov 11, 2018 #11


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    Even a removed message casts a shadow in the thread and so I think it’s time to close this thread.

    Thank you all for contributing here.

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