I am a deep admirer of natural sciences, and especially physics, but, alas, only at a lay level; so I'll ask forgiveness in advance for my poor terminology and half-baked understanding of the fundamentals that I'm asking about.

I seem to recall a lecturer saying something to the effect of "light prefers to stay in its existing medium by bouncing internally within that medium (and some of it exits, refracted, into the new medium)." I seem to recall that he demonstrated this by shining a handheld laser light through a rectangular glass frame, with the audience able to see that the laser light bounced internally within the glass where it hit the boundaries of the glass frame.

This made me wonder whether I could create a poor-man's version of the experiment by having directional light "follow" a flowing stream of water -- because (per my imperfect understanding) once light is in water, it will "prefer to stay in its existing medium.".

I've a diagram of my experiment:

enter image description here

I don't have a laser light, but I have a flashlight with a pretty tight focus. I shine it through plastic wrap covering a hole cut out on a milk carton that's filled with water.
I wondered:
Light should be bouncing around inside the carton (I know that nothing is 100% reflective, so some of the light will be absorbed, or otherwise "lost")...so when I open the spout, and let a stream of water flow, will some of the bouncing light make its way through the path of the water flowing out the spout to the ground?
I.e. will there be an appreciable percentage of light from the flashlight that, after bouncing around inside the carton, will make its way to the start of the flowing stream of water (the spout opening)...and once in the stream of flowing water, will an appreciable percentage of light "prefer to stay in the existing medium" by bouncing internally...some percentage of which might bounce all the way to the end of the stream of flowing water: the ground?

Having done the experiment in real life, there does appear to be a spot of light where the stream of water hits the ground -- with varying intensity. A couple of photos:
enter image description here enter image description here enter image description here
...but having only a lay appreciation of physics, I am likely prone to misinterpreting observation, confirmation biases, etc.

In summary, my questions are: how correct is my notion that light "prefers to stay in its existing medium", and is that the reason for the spot of light at the end of the stream of water in my experiment? If not, what is the correct explanation for that perceived spot of light at the end of the stream of water?

  • $\begingroup$ If you haven't yet, optic fibers are something to look into! I don't know about your specific experiment you share though $\endgroup$ Jun 17, 2022 at 21:53

1 Answer 1


Light can follow a stream of water by internal reflections between the water and air boundaries. The critical angle for internal reflection between water and air is approx. 48.5° so any light in the water hitting the air/water boundary with an angle less than the critical angle will propagate through.


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