| I replaced the headlights in my Z today, mine are old and I was looking for something brighter. After pulling them out, I noticed that the filament was in pretty bad shape. They still worked but the filament looked dull and splintered. Also I could see little shards of filament on the inside of the bulb. I thought it would be neat to have a closer view of an old filament so here we go: Here it is at 10x, you can see the splinters of tungsten that have flaked off the filament onto the glass. You can also see the irregular texture on the filament itself.
 At 60x, you can see how the filament is in bad shape. Its surface is chipped all throughout. Tungsten is brittle and many heating/cooling cycles will do that. I shined a light from the side to highlight the surface details.
 At 200x my toy microscope's optical resolution takes a major hit. It's pretty blurry and dark from the magnification but you can still see the major chips that are missing from the filament.
 Each time you turn a light on or off the temperature changes thousands of degrees in an instant. That's usually when these chips break off, when it's heating up or cooling down. When the chips break through the whole filament, the bulb burns out. That's also why bulbs rarely burn out while they're on, they usually burn out when you first turn them on and the filament is undergoing the most stress. That's my little science class for the day. Now get out of my classroom.
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Closeup pics of my Z's old headlights - 
