A goldfish isn't always visible inside its fishbowl. If the fish gets sufficiently close to the bowl glass, it will disappear for certain angles of observation. In a recent paper in this journal,1 Zhu and Shi explained the effect in terms of total internal reflection. We show in what follows that the phenomenon involves some richer optics: a rainbow defines one of the boundaries of the “blind zone” where the fish disappears.

1.
Zhu
Yuhua
and
Shi
Fengliang
,
“Why does the goldfish disappear in the fishbowl?”
Phys. Teach.
47
,
424
426
(Oct.
2009
).
2.
Tabulae, dynamic geometry software, http://tabulae.net.
3.
C. E.
Aguiar
,
“Óptica e geometria dinâmica,”
Revista Brasileira de Ensino de Física
31
,
3302
(
2009
).
4.
R. J.
Whitaker
,
“Physics of the rainbow,”
Phys. Teach.
12
,
283
286
(May
1974
).
5.
J. D.
Walker
,
“Multiple rainbows from single drops of water and other liquids,”
Am. J. Phys.
44
,
421
433
(May
1976
).
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