Why do bulbs get dimmer in series circuits?

When light bulbs are connected in series circuits, they will get dimmer whenever a bulb is added. This is because the current is shared between all of the bulbs equally.

Answer

Lamps only operate at their rated power when subjected to their rated voltage. If you connect lamps in series, each lamp will be subject to a lower voltage, so they will not operate at their rated power and, so, will be dimmer. The more powerful lamps will actually glow less than the less-powerful lamps because their resistance is lower and the voltage drop across them will also be lower than the voltage drop across the less powerful lamps.