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How does a light bulb work?

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Quick answer

An incandescent light bulb works by passing electricity through a thin tungsten filament, which resists the current and heats up. At around 2,500°C the filament glows and emits light. The glass bulb holds a vacuum or inert gas to stop the hot filament from reacting with oxygen and burning out.

An incandescent light bulb works by passing electric current through a thin tungsten filament, heating it until it glows. The glass bulb holds a vacuum or inert gas to prevent the filament from burning up.

Step by step

  1. Electric current enters through the metal base and flows into the filament.
  2. The filament's resistance converts electrical energy into heat.
  3. The filament temperature rises until it becomes incandescent, emitting light.
  4. The glass bulb's vacuum or inert gas prevents the filament from oxidizing.
  5. Light passes through the glass, and the bulb slowly loses tungsten over time.

Key things to understand

  1. Electric current flows from one contact, through the filament, to the other contact.
  2. The filament resists the current, converting electrical energy into heat.
  3. As the filament heats to about 2,500°C, it emits visible light.
  4. The glass bulb contains inert gas or a vacuum to stop oxidation.
  5. Only about 5% of the energy becomes light; the rest is wasted as heat.

Labelled diagram

  1. Glass bulb

    Sealed glass shell that holds vacuum or inert gas around the filament.

  2. Filament

    Thin tungsten wire that glows white-hot when current passes through.

  3. Support wires

    Metal wires that hold the filament in place and carry current to it.

  4. Base

    Metal screw base that connects the bulb to the electrical socket.

Parts of an incandescent light bulb

Real-world examples

  • A standard 60-watt incandescent bulb in a table lamp uses a tungsten filament that glows yellow-white.
  • Halogen bulbs, used in car headlights, contain halogen gas that redeposits tungsten back onto the filament.
  • A flashlight bulb has a small filament and a glass envelope filled with inert gas.
  • Early carbon-filament bulbs, developed by Thomas Edison and Joseph Swan around 1879–1880, burned out far sooner than later tungsten filaments.

Common mistake

✗ Myth: A light bulb creates light without any heat.

✓ Fact: Incandescent bulbs produce light as a byproduct of heating a filament to incandescence. About 95% of the energy is released as heat, which is why they are hot to touch.

People also ask

What makes a light bulb glow?
Electric current flows through a thin tungsten filament inside the bulb. The filament resists the current, heating up to around 2,500°C. At that temperature it emits visible light, glowing brightly. The glass bulb keeps oxygen away so the filament does not burn up.
Why do light bulbs get hot?
Incandescent bulbs get hot because they work by heating a filament until it glows. Most of the electrical energy, about 95%, turns into heat rather than light. That heat radiates out through the glass, making the bulb too hot to touch.
What gas is inside a light bulb?
Most incandescent bulbs contain an inert gas such as argon or nitrogen, sometimes mixed with a little krypton. The gas prevents the hot tungsten filament from reacting with oxygen and burning out. Some bulbs use a vacuum instead, but inert gas slows evaporation of the filament.
Why do light bulbs burn out?
The tungsten filament slowly evaporates at high temperatures, thinning over time. Eventually a thin spot breaks, creating a gap that stops the current. Vibration or a sudden surge of electricity can also snap the filament.
Why is the glass of a light bulb frosted or white?
Frosting or a white coating diffuses the bright, harsh light from the filament. This spreads the light more evenly and reduces glare. Clear bulbs give a sharper light with more glare.
Can you touch a halogen bulb with bare hands?
No, because skin oils can create hot spots on the quartz glass. Those spots may cause the bulb to crack or shatter when it heats up. Always handle halogen bulbs with a cloth or gloves.
Why do LED bulbs use less electricity than incandescent bulbs?
LEDs produce light directly from electrons moving through a semiconductor, so they waste very little energy as heat. Incandescent bulbs heat a filament until it glows, losing most energy as heat. That is why LEDs convert a much larger share of electricity into light.

Part of the guide: Everyday Machines →