A fluorescent bulb is a glass tube usually filled with argon gas and a little bit of mercury. When turned on, the cathode heats up and sends out electrons. These hit the argon gas and the mercury. The argon gas makes a plasma which lets the electrons move around better. When the electrons hit a mercury atom it puts the molecule into a state where it has a lot of energy (stores the energy). The energetic state doesn't last very long, and when the energy is released, it lets out a photon. Photons from mercury are not visible like some other photons; they are ultraviolet. So there's a phosphor coating on the wall of the bulb. When the photon hits a phosphor molecule, it in turn puts that molecule into an excited state. When this phosphor releases energy, it lets out a photon that we can see, and light is made. Changing the type of phosphor can change the color we see, but usually fluorescent light bulbs are whiter than incandescent light bulbs, which are slightly yellow.