Varying the voltage of a light bulb

[sanado]

Hey guys, was wondering if you could help me with this question:

What determines the brightness of a lightbulb?

I think it would be power (and therefor voltage and current since P = VI) as power is used in the measurement of energy delivered. Why is it the case then that, from the questions i am doing, it states that if you have a simple circuit such as the one shown in the following link (page 11, question 5), why doesn't the brightness vary when one part of the parrallel circuit is removed?

http://www.vcaa.vic.edu.au/vce/studies/physics/pastexams/physics12004.pdf

Question 5, page 11

Thanks

 

[Mapes]

I don't see the contradiction. There's no power associated with the removed bulb, so it's dark. The power is unchanged for the others*, so their brightness is unchanged.

*P = VI = V²/R, and voltage and resistance are constant.