The message
from Janet Baraclough contains these words:

Sometimes real life contradicts physics theory :-).I have a
collection of those clear solid glass spheres with a few air bubbles
inside the glass. When we moved here I set them on a sunny south
windowsill, until I found that when the sun shines through the window
and through the glass balls, there's a focussed hotspot; hot enough that
you wouldn't want to keep your hand there for long.

But the refractive index of glass is much higher than that of water.

--
Tony
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