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Phosphor glows: Why, What, How long
![[Charles Hoberman, used with permission] The glow-in-the-dark Hoberman Mini Sphere](images/2003-04-18-glow-in-dark.jpg) Q: Why do phosphors glow?
—Physics student
[Charles Hoberman, used with permission] The
glow-in-the-dark Hoberman Mini Sphere
A: Phosphors glow because they have a peculiar property: shining
ultraviolet light on a phosphor excites the electrons in their atoms so the
electrons jump to a higher energy state. Furthermore, the electrons get "stuck"
in this higher state because it’s extremely difficult for them to radiate light
and thereby lose the extra energy. If you turn off the light, the electrons
randomly radiate light and slowly lose energy (some phosphors taking as long as
several hours). That’s why they glow in the dark and gradually dim.
Q: You said that clothes glow in "black light"
because of phosphors in laundry detergent. Can objects glow because of a
substance other than phosphor? What other objects have phosphors? —Alex
N, Forest Lake, Maine
Only phosphors glow under ultraviolet (or higher energy) light, by
definition, because such substances are called phosphors.
Many objects contain phosphors: some phosphorescent crystals and large
organic molecules occur naturally. Chemists have created thousands of such
chemicals: zinc sulfide and strontium aluminate are a couple that toy makers
use. Your TV screen glows because of phosphors that decay just slowly enough
that successive pictures blend into each other. Fireflies, some bacteria, and
even jellyfish chemically excite molecules in their bodies that give off light.
These creatures are called bioluminescent, a related and somewhat mysterious
phenomenon.
Q: If you hold an item in front of the light for
10 minutes, will it glow twice as long as for 5 minutes? —Debbie B’s
5th grade daughter
Phosphors will not glow longer if held under a light for a longer time. The
critical thing is the level of energy the light source emits. A phosphor
electron must receive a certain minimum photon energy level (ultraviolet
radiation) before it can jump to a higher energy state. If your source has
insufficient energy, you could hold the item under it all day and not get any
glow. Whereas, you could hold the same object under an ultraviolet light for
just long enough (a few seconds), and see it glow.
Further Surfing:
Sanjay
Rebello, Kansas State University: Phosphorescence
WonderQuest: Black light
makes phosphorescent detergent powders glow
WonderQuest: It’s not fire that lights fireflies
(Answered March 18, 2003)
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