When plane-polarised light with amplitude $a$ is sent through a polarising filter, as illustrated, the amplitude of the light that emerges is $a \cos \theta$. The intensity of the incident beam is $I$. What is the intensity of the emerging light when $\theta$ is $60.0^\circ$?
- A$0.250\,I$
- B$0.500\,I$
- C$0.750\,I$
- D$0.866\,I$