Lightbulbs

Earlier today my dad came into my room to give me new lightbulbs because he took mine the other day. I got hands on and started changing my lightbulbs, and I started thinking about lighting class! (Cheesy, but true.) My lightbulbs are A-type and they have a white powder on the inside of the glass so the filament isn't seen. I looked at the old lightbulb and I realized that the powder was fading and at one spot on the bulb it was even gone. It made me wonder how that happens. At least now, I have light in my room!