This is a picture of my hand held Tesla coil that we were experimenting with today. The arc between the tip and the pear is emitting light because electrons that have been shot off the tip collide with an electrons orbiting atoms. When they collide the orbiting electrons jump up to a high energy state and absorb an electron, then they fall back down emitting a photon. That is why you can see the arc. The arc is also made up of plasma, which is a cloud of free floating electrons that whiz around. Sometimes the flying electrons smash into an oxygen molecule and split it apart into two oxygen atoms. If an oxygen atom runs into an oxygen molecule they combine to form an ozone molecule O₃ which is three oxygen atoms bonded together.

I had fun experimenting with the Tesla coil with my classmates.