Excitation of electrons.
<P>
   When  a  chlorophyll  pigment absorbs a photon of light, electrons are
   excited to an unstable high-energy state.
<P>
   When  the  electrons  fall  back  to  the  ground state, the energy is
   released.
<P>
   This   energy   can  be  visualized  by  illuminating  a  solution  of
   chlorophyll and observing the light (fluorescence) and heat.
<P>
   A spinach battery:  <A ClAsS=MID Href="../../../med/Vid/Johnson4e/VideoQuiz/spinachbattery_pur5_ISDN.mov" oNClick="popurlt('../../../../med/Vid/Johnson4e/VideoQuiz/spinachbattery_pur5_ISDN.mov');txtcolor('excite1','gray');return false;" Id=excite1><iMg sRc="/images/quicktime.gif" aLt='' boRDer=0></A>