What factor is most likely responsible for the spike in europe's population that took place from about 1800 to 1950?

Can someone explain to me how to do this problem ?

An initially unpolarized light if intensity 500 w/m^2 passes through four polarizers. the transmission axis between adjacent polarizers is 45∘. what percentage of the initial intensity is transmitted by the system?

Ascientific blank must be testable and capable of being proven false?