system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do lightbulbs take a small moment to power on in certain cases, but turn off instantly?
gpt
Fluorescent light bulbs (and Compact Fluorescent bulbs) need to do a couple of things before visible light is produced.  A high voltage electrical arc passes through the tube, which separates electrons from their parent atoms in the gas in the tube.  This creates a situation where a lower current can then travel through the gas (since the electrons are now free to move about).  That allows UV light to be produced, which excites the phosphor coating on the inside of the tube, generating visible light.  
   
The arcing process is what takes a moment.  
  
Incandescent bulbs need to heat up the filament to glow white hot, but that takes only fractions of a second normally.  And the mechanism used in LED bulbs is even faster.