How do 'fiber-optic' gun sights work?

[nabiul]

I have thought about it and still can't seem to figure this out. How do those nifty plastic rods channel the light entering from the sides to the ends? All of the theory I know suggests that the light striking the sides would pass right through after being displaced from refraction, UNLESS the sides of the plastic object were slanted in a prism like structure which would make it a 'one-way' light channel. But the plastic rods used in gun and bow sights are just regular cylindrical rods from what I can tell.

Any ideas?

 

[carrot]

Magic!

I just surmised that any light entering from one side of the rod comes out the other, hence making it appear to "light up" in low light conditions.

 

[nabiul]

Thats not it, if you've seen them, they glow as bright as opaque LEDs in regular daylight. If that was the case, any old clear material would work, but it doesn't. This also has to do with ruby lasers, how does light coming perpendicular from a flashlamp paralell to the ruby rod end up going out through the ends of the rod?

AUGHHHH, my head, I need to understand this.

 

[TaschenlampeMann]

I believe this phenomenon results from the differences in refractive index of the plastic material and air. In general, light can enter the cylinderical sides of the plastic from many angles, but once inside some of the rays will be "trapped" inside unable to exit because they strike the inside surface at too low an angle. Those rays bounce around inside the plastic till they encounter a surface at a more nearly right angle (the end faces) where they can exit making that surface appear bright.

The physcists among us will, no doubt, be able to supply a better explanation.

Hope this helps.

 

[nabiul]

Nope thats not it either, if you do the math you will see that any ray entering from the sides will never strike the second inside surface at an angle greater than the critical angle for total internal reflection. The critical angle is when the transmitted ray is parallell to the material surface, all light from outside is at a greater angle.

AUGHH, I forgot one thing as mentioned on another forum. For every refraction there is a reflection, before the beam exits the material on the other side, a portion of that beam is reflected back inside as a specular reflection, which continues to reflect inside. However this isn't exactly it either, since only a portion of the light at each reflection is reflected, within a few reflections all of the light that came in will leave.

Theres more to this.

 

[bobski]

Something to do with fluorescence of the light channel material? All the sights I've seen have been made of some kind of day-glo plastic, and seem to work far better under light sources that trigger the day-glo effect.

Here's the sight on my bow, lit up by an incandescent light. Note the brightness of the green sight point.

The same pic, but with the sight lit by a black light. Note the green sight point is the same brightness, in spite of the fiber being much more dimly lit.