Modern Physics > Atomic Physics DCS# 7B13.50
PHOSPHORESCENT MATERIALS

APPARATUS
gooseneck lamp and/or UV lamp
 101-16-B
one or more of these phosphorescent samples:
    phosphorescent paper
 202-21-C5
    glow string
 202-17-A3
    fluorescein powder
 202-21-D6
    clock with radium dial 202-04-D6

DESCRIPTION
Place the phosphorescent items under bright white or uv light for a few seconds, then dim the room lights to show that they glow in the dark for some time.  Phosphorescence is observed when transitions take place between states with different spin multiplicities.  These transitions are forbidden by quantum mechanics, so the probability for the transition is small and the lifetime of the state is long.

To show that light of sufficiently high frequency is necessary to excite the phospors, shine a red and/or green laser and a blue LED on the phosphorescent paper.  The low frequency lasers leave no trace, whereas the blue LED can be used to "write" on the paper.