Find out what makes the bulb dimmer
It is interesting to look in more detail at what happens in the circuit when an extra bulb is added.
With an extra bulb, the total resistance in the circuit is increased so the charged particles move around more slowly. This results in:
collisionshifts less energy to the ions) in both bulbs.
So, both bulbs in the circuit become dimmer as a result of the second bulb being added because fewer charged particles per second pass through each bulb and the interactions between charged particles and lattice ions are less energetic.