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Physics/Refraction and Parabolics


QUESTION: How do I calculate the angle of refraction of parallel sun rays through a polycarbonate tube 1.5OD, 1.25ID? Specifically it's in relation to a trough type parabolic mirror that has a copper tube as the receiver and the polycarbonate tube as the copper tube's sleeve.

ANSWER: I'm going to assume that you mean something that could be graphically represented as =O (viewing the tube end-on, where the O represents the tube and where the = sign is supposed to look like parallel rays of light).  You can't calculate just one angle, the angle will depend on how far from the center of the tube the light rays hit it.  If you have that information, you can use Snell's law and geometry to construct the angle of refraction for the ray as it passes into the tube, then more geometry to figure out the angle where it passes out of the tube, then use Snell's law and yet more geometry to figure out the angle of refraction for the exit.  That's a laborious problem, even with that information!

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Parabolic Refraction Question
Parabolic Refraction Q  
QUESTION: Thanks so much!
I am more concerned with the entry refraction, the exit is absorbed by a copper pipe. And as I read your answer, I realized I am more concerned with calculating the refraction angle's direction,as in a minus or plus direction (towards or away from the copper pipe). As I build bigger mirrors the variance in accurate focus point is bound to occur and the rays of light while they may still strike the receiver, may not strike the plexi tube sleeve perpendicularly, .... where I am supposing the light may refract. Attached is an illustration.
Thanks again for your quick and clear explanation to my question,

ANSWER: Oh, this will indeed work...but it would be better if you could fill the intervening space with water, that would help focus the light.

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my parabolic model
my parabolic model  

two parabolic shapes
two parabolic shapes  
QUESTION: That would be great, but just with my little 24" model I'm generating temperatures inside the copper tube in excess of 335F (photo attached).
... So what happens to that contained water then? ... Or is there a really clear antifreeze to use?
I'm just finishing up a larger homemade unit with about 3 times the linear surface area directed to the same size receiver. What would the expected increase in temperature be?
...And does deeper and shallower parabolic curves affect efficiency, or does one require a a higher degree of shape integrity and focal placement accuracy? Attached are examples of two very different parabolic shapes, with equal linear surface.
Thank you so much for your time and attention, ...

Your pipe is not parabolic, it's circular.  That's hot, you'd need pressurized water to keep it from boiling.  Antifreeze is not a good answer, you'd need mineral oil.  Would work better, anyhow.  Would not boil.  Might melt the plastic, though.  You won't need to worry about circular aberration in this close of a geometry.  Linear surface area does not mean much.  Diameter is the important part.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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