Water pearls are superabsorbent polyacrylate beads that can expand about 200 times when submerged in water and are often used for decorative purposes in flower vases. A bag of pearls costs about a dollar. The pearls are very useful for teaching science, especially optics.1 Since water pearls are mainly made of water, their index of refraction matches the index of refraction of water. Consequently, they become invisible while submerged in it (Fig. 1). A spherical shape and the transparency of water pearls make them almost “perfect” spherical water lenses (Fig. 2).

1.
G. R.
Gore
, “
Physics fun with jelly marbles
,”
Phys. Teach.
47
,
10
(
Dec. 2009
).
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