Recently, someone told me to pay attention when hanging a wind chime containing a glass sphere in front of the window, because a focus point might be formed on the curtains which might on a sunny day set the curtains on fire.
To my intuition, this seemed wrong, but I am unsure if my intuitive feeling is correct.
I know that because of the fundamental property of a sphere/circle, any light ray going through its center will pass through unrefracted.
Also, I read about Spherical Abberation, which seems to indicate that (at least parallel) light rays will not end up in the same focus point.
I believe that incoming rays of sunlight are usually considered to be parallel.
Is this enough to say that burning your curtains by having sunlight pass through a glass sphere is highly unlikely, or not?
A more formal explanation of why a burning point might (or cannot) be formed in this scenario would be greatly appreciated.
Best Answer
Ball lenses work just fine (http://www.edmundoptics.com/resources/application-notes/optics/understanding-ball-lenses/). In ordinary lenses, light rays going through the optical center of the lens are not deflected either. Ordinary lenses also have spherical aberrations. I don't have a ball lens at home, but I am pretty sure one can start a fire using such a lens. See also https://www.exploratorium.edu/snacks/water-sphere-lens