To solve this problem it is necessary to apply the concepts related to the Law of Malus for which it is understood that
[tex]I=I_{0}\cos ^{2}\theta _{i}[/tex]
Where
[tex]I_{0}[/tex] indicates the intensity of the light before passing through the polarizer,
I is the resulting intensity, and
[tex]\theta _{i}}[/tex] indicates the angle between the axis of the analyzer and the polarization axis of the incident light.
The intensity of light after passing through a series of three polarisers would be given as
[tex]I_3 = \frac{I_0}{2}cos^2\theta_1*cos^2\theta_2[/tex]
Here,
[tex]\theta_1 = 30\°[/tex]
[tex]\theta_2 = 90-60 = 30\°[/tex]
Therefore [tex]\theta_1 = \theta_2[/tex]
[tex]I_3 = \frac{I_0}{2}cos^2(30)*cos^2(30)[/tex]
[tex]I_0 = \frac{2I_3}{cos^430}[/tex]
[tex]I_0 = \frac{2*380}{cos^430}[/tex]
[tex]I_0 = 1.35*10^3W/m^2[/tex]
Therefore the original intensity of light was [tex]1.35*10^3W/m^2[/tex]
