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An incandescent bulb has a thin tungsten filament heated to high temperature by an electric current; the hot filament emits black-body radiation. Over a long time the filament breaks at random spots because tungsten evaporates non-uniformly. The bulb is run at constant voltage. Which statement(s) is/are correct?

1. The temperature is the same everywhere along the filament.
2. The resistance of small thinned sections of the filament decreases with time.
3. Just before breaking, the filament emits relatively more light at higher frequencies.
4. Near the end of the bulb's life the filament draws less electrical power.

Correct answer: Just before breaking, the filament emits relatively more light at higher frequencies.

Solution

Non-uniform evaporation makes some sections thinner with higher resistance, so temperature is non-uniform; the hotter thinned regions shift their emission peak to higher frequencies. (Options about decreasing section resistance and uniform temperature are false; whether total power drops depends on the model, but the higher-frequency emission statement is clearly true.)

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