) What is the difference between fluorescence and phosphorescence?

Answer 1

The simplest demonstration of the difference between
fluorescence and phosphorescence is to look at a black
light poster (under black light) and to look at
anything that's "glow in the dark" (like those star
stickers you put on ceilings or the hands on a watch).
The posters only "glow" under the black light--the
second you turn off the light, the glowing goes away.
The posters have paint that fluoresce. In contrast,
the hands on a watch continue to glow for a long time
after the light source was removed. The watches are
painted with a paint that phosphoresces.

This is the simple demo of the two phenomena, now
let us examine why these examples behave the way they
do.

Fluorescence and phosphorescence describe two
different ways that an electron can behave in a
molecule. They start the same way: a molecule absorbs
light, and an electron gets excited to a higher energy
level. If the electron drops straight back down to
where it started from without undergoing any spin
changes (this excited state is called a "singlet
state"), then you have fluorescence. The electron
admits energy in the form of a photon (light) on its
way back down.

In contrast, when an electron gets excited and its
spin DOES flip (this is called a "triplet" state) it
takes much longer for it to release the energy and
come back down to the ground electronic state.
This is phosphorescence. Things that phosphoresce
"glow" for much longer because the excited electrons
take so much longer to come back down to their initial
energies.

So, the next time you see a black light poster or
glow-in-the-dark watch hands,