In a circuit with one bulb, another bulb is added in series. What happens to the bulbs when current is passed?
They glow less brightly.
A circuit has one bulb, if another is connected in series, the voltage(potential difference) will be divided across both the bulb,
i.e., if the voltage across the first bulb was V then the new voltage across the 1st and the 2nd bulb will be V1 and V2, such that V= V1 + V2.
Because of the potential drop/ divide, the brightness of the two bulbs will be less than the brightness with which the single bulb was glowing earlier.