Question

What is the origin of dark spots on the inner glass walls of an old tungsten lightbulb? What is the purpose of filling these lightbulbs with argon gas?

Short answer

The dark spots on the inner glass walls of an old tungsten lightbulb appear due to the sublimation of the tungsten filament over time. The lightbulbs are filled with argon gas to slow down this sublimation process by inhibiting the rate of filament oxidation, thus prolonging the bulb's lifespan.

Step-by-step solution

Understanding Tungsten Filament

When the tungsten light bulb is used, the electricity heats up the tungsten filament inside. The heating of the filament causes it to glow and give out light. Over time, the tungsten filament experiences a process called sublimation, which is the transition of a substance directly from the solid phase to the gas phase. Tiny parts of the filament sublime and can then stick to surrounding surfaces, here, the inner glass wall of the bulb, creating dark spots.

Argon's Purpose

Argon is an inert or noble gas with very low reactivity. This lack of reactivity makes it an ideal choice for use in lightbulbs. Filling the bulb with argon gas slows down the sublimation process of the tungsten filament. It does so by reducing the rate at which the filament gets oxidized, ensuring a longer lifespan for the bulb.