question: How do light bulbs go out?
comment: If you mean "burn out": the tungsten filament inside a bulb is relatively fragile. When you turn the bulb on and off the filament heats up then cools down. Each heat/cooling cycle will put stresses on the material and over time it will develop weak spots until the material just "breaks" in a spot. Bulbs that turn on/off frequently (like in your entrance hallway, or a bedside reading lamp) will (in general) burn out quicker than ones that are infrequently turned on or are left on for long periods. Of course the bulbs for special purposes like your car headlights or the bulb in your fridge are designed to different robustness standards.
passages: In most modern incandescent bulbs, part of the wire inside the bulb acts like a fuse: if a broken filament produces an electrical short inside the bulb, the fusible section of wire will melt and cut the current off to prevent damage to the supply lines.
  
  A hot glass bulb may fracture on contact with cold objects. When the glass envelope breaks, the bulb implodes, exposing the filament to ambient air. The air then usually destroys the hot filament through oxidation.
  
  Section::::Physical characteristics.:Bulb shapes.
   Another reason for the longevity of bulbs is the size, quality and material of the filament.
  
  Section::::Other long-lasting light bulbs.
  
  Section::::Other long-lasting light bulbs.:Second.
   The light bulb is supplied with electric current by feed-through terminals or wires embedded in the glass. Most bulbs are used in a socket which provides mechanical support and electrical connections.
   Christmas lights using incandescent bulbs are somewhat notorious for being difficult to troubleshoot and repair. In the 1950s and 1960s, the series circuit connected light sets would go completely dark when a single bulb failed. So in the fairly recent past, the mini-lights have come with shunts to allow a set to continue to operate with a burned out bulb. However, if there are multiple bulb failures or a shunt is bad, the string can still fail. There are two basic ways to troubleshoot this: a one by one replacement with a known good bulb, or by using a test light to find out where the voltage gets interrupted. One example made specifically for Christmas lights is the LightKeeper Pro. For additional information, see the troubleshooting section of the Holiday lighting technology article.
   In most CFLs the filaments are connected in series, with a small capacitor between them. The discharge, once lit, is in parallel to the capacitor and presents a lower-resistance path, effectively shorting the capacitor out.
  
  Section::::Principles of operation.:End of life.:Phosphor.
   The Ribbon Machine works by passing a continuous ribbon of glass along a conveyor belt, heated in a furnace, and then blown by precisely aligned air nozzles through holes in the conveyor belt into molds. Thus the glass bulbs or envelopes are created. A typical machine of this sort can produce anywhere from 50,000 to 120,000 bulbs per hour, depending on the size of the bulb. By the 1970s, 15 ribbon machines installed in factories around the world produced the entire supply of incandescent bulbs. The filament and its supports are assembled on a glass stem, which is then fused to the bulb. The air is pumped out of the bulb, and the evacuation tube in the stem press is sealed by a flame. The bulb is then inserted into the lamp base, and the whole assembly tested. The 2016 closing of Osram-Sylvania's Wellsboro, Pennsylvania plant meant that one of the last remaining ribbon machines in the United States was shut down.
   Section::::Principles of operation.:End of life.
  
  The end of life failure mode for fluorescent lamps varies depending on how they are used and their control gear type. Often the light will turn pink (see Loss of mercury), with black burns on the ends of the lamp due to sputtering of emission mix (see below). The lamp may also flicker at a noticeable rate (see Flicker problems).
  
  Section::::Principles of operation.:End of life.:Emission mix.
   The fifth-longest-lasting light bulb was in a washroom at the Martin & Newby Electrical Shop in Ipswich, England. Given its shape and design, it was thought to date from the 1930s. It burned out in January 2001.
  
  Section::::Edison and the Eternal Light.
   Lamp bases may be secured to the bulb with a cement, or by mechanical crimping to indentations molded into the glass bulb.
  
  Miniature lamps used for some automotive lamps or decorative lamps have wedge bases that have a partial plastic or even completely glass base. In this case, the wires wrap around to the outside of the bulb, where they press against the contacts in the socket. Miniature Christmas bulbs use a plastic wedge base as well.
   The Divine Comedy's song "When The Lights Go Out All Over Europe," from the "Promenade" album (1994), is a paean to the cinema stars of the 1940s and '50s.
  
  In the final episode of the third series of "That Mitchell and Webb Look", a sketch depicts the show closing down due to lack of funds (because the show was illegally pirated). At the end of the sketch David Mitchell (in character as himself) declares tearfully "The lights are going out all over television, we shall not see them lit again in our lifetime!"
   The phosphor drops off in efficiency during use. By around 25,000 operating hours, it will typically be half the brightness of a new lamp (although some manufacturers claim much longer half-lives for their lamps). Lamps that do not suffer failures of the emission mix or integral ballast electronics will eventually develop this failure mode. They still work, but have become dim and inefficient. The process is slow, and often becomes obvious only when a new lamp is operating next to an old one.
  
  Section::::Principles of operation.:End of life.:Loss of mercury.
   Headlamp systems require periodic maintenance. Sealed beam headlamps are modular; when the filament burns out, the entire sealed beam is replaced. Most vehicles in North America made since the late 1980s use headlamp lens-reflector assemblies that are considered a part of the car, and just the bulb is replaced when it fails. Manufacturers vary the means by which the bulb is accessed and replaced. Headlamp aim must be properly checked and adjusted frequently, for misaimed lamps are dangerous and ineffective.
   Although such failure is associated with end of life, an arc tube can fail at any time even when new, because of unseen manufacturing faults such as microscopic cracks. However, this is quite rare. Manufacturers typically "season" new lamps to check for such defects before the lamps leave the manufacturer's premises.
   One example is in miniature Christmas lights which are wired in series. When the filament burns out in one of the incandescent light bulbs, the full line voltage appears across the burnt out bulb. A shunt across the burnt out filament will then short out to bypass the burnt filament and allow the rest of the string to light. If too many lights burn out however, a shunt will also burn out, requiring the use of a multimeter to find the point of failure.
  
  Section::::Photovoltaics.
   Section::::Construction.
  
  Incandescent light bulbs consist of an air-tight glass enclosure (the envelope, or bulb) with a filament of tungsten wire inside the bulb, through which an electric current is passed. Contact wires and a base with two (or more) conductors provide electrical connections to the filament. Incandescent light bulbs usually contain a stem or glass mount anchored to the bulb's base that allows the electrical contacts to run through the envelope without air or gas leaks. Small wires embedded in the stem in turn support the filament and its lead wires.
   It is useful to consider the common experiences we have with light bulbs. Light bulbs "burn out" more or less at random; eventually the repeated heating and cooling of its filament, and fluctuations in the power supplied to it cause the filament to crack or vaporize. The same principle applies to most other electronic devices and similar principles apply to mechanical devices. Some failures are part of the normal wear-and-tear of components in a system.
   As in all mercury-based gas-filled tubes, mercury is slowly adsorbed onto the glass, phosphor, and tube electrodes throughout the life of the lamp, until it can no longer function. Loss of mercury will take over from failure of the phosphor in some lamps. The failure symptoms are similar, except loss of mercury initially causes an extended run-up time to full light output, and finally causes the lamp to glow a dim pink when the mercury runs out and the argon base gas takes over as the primary discharge.
   The "emission mix" on the lamp filaments/cathodes is required to enable electrons to pass into the gas via thermionic emission at the lamp operating voltages used. The mix is slowly sputtered off by bombardment with electrons and mercury ions during operation, but a larger amount is sputtered off each time the lamp is started with cold cathodes. The method of starting the lamp has a significant impact on this. Lamps operated for typically less than 3 hours each switch-on will normally run out of the emission mix before other parts of the lamp fail. The sputtered emission mix forms the dark marks at the lamp ends seen in old lamps. When all the emission mix is gone, the cathode cannot pass sufficient electrons into the gas fill to maintain the gas discharge at the designed lamp operating voltage. Ideally, the control gear should shut down the lamp when this happens. However, some control gear will provide sufficient increased voltage to continue operating the lamp in cold cathode mode, which will cause overheating of the lamp end and rapid disintegration of the electrodes (filament goes open-circuit) and filament support wires until they are completely gone or the glass cracks, destroying the low pressure gas fill and stopping the gas discharge.
   If the lamp is installed where it is frequently switched on and off, it will age rapidly. Under extreme conditions, its lifespan may be much shorter than a cheap incandescent lamp. Each start cycle slightly erodes the electron-emitting surface of the cathodes; when all the emission material is gone, the lamp cannot start with the available ballast voltage. Fixtures intended for flashing of lights (such as for advertising) will use a ballast that maintains cathode temperature when the arc is off, preserving the life of the lamp.
   Section::::Principles of operation.:End of life.:Ballast electronics.
   Section::::American Institute of Electrical Engineers.:Case 2.
   Section::::Principles of operation.:Electrical aspects of operation.
  
  Fluorescent lamps are negative differential resistance devices, so as more current flows through them, the electrical resistance of the fluorescent lamp drops, allowing for even more current to flow. Connected directly to a constant-voltage power supply, a fluorescent lamp would rapidly self-destruct because of the uncontrolled current flow. To prevent this, fluorescent lamps must use an auxiliary device, a ballast, to regulate the current flow through the lamp.
   Section::::Background.
   Section::::American Institute of Electrical Engineers.:Case 1.
   Another phenomenon associated with HID lamp wear and aging is discoloration of the emitted light beam ("fading"). Commonly, a shift towards blue and/or violet can be observed. This shift is slight at first and is more generally a sign of the lamps being "broken in" whilst still being in good overall working order, but towards the end of its life, the HID lamp is often perceived as "only" producing blue and violet light. Based on Planck's law, this is a direct result of the increased voltage and higher temperature necessary to maintain the arc.
  