Answer
a) $0.5403~W/m^2$
b) $272~mW/m^2$
Work Step by Step
(a) We know that
$I=\frac{1}{2}I_{\circ}$
$I=\frac{1}{2}(1.60W/m^2)$
$I=0.8W/m^2$
The new intensity at point B is given as
$I^{\prime}=Icos^2 25^{\circ}$
$I^{\prime}=(0.8W/m^2)(0.821)$
$\implies I^{\prime}=0.657W/m^2$
Now the intensity at point C is
$I^{\prime \prime}=i^2cos^2(25^{\prime})$
$\implies I^{\prime \prime}=(0.657)(0.821)=0.5403W/m^2$
(b) We know that
$I=\frac{1}{2}I_{\circ}$
$\implies I=\frac{1}{2}(1.60W/m^2)$
$\implies I=0.8W/m^2$
The intensity at point B is
$I^{\prime}=Icos^2 50^{\prime}$
$I^{\prime}=(0.8W/m^2)(0.41317)=0.330W/m^2$
Now the new intensity at point C is
$I^{\prime \prime}=I^{\prime} cos^2(25^{\circ}-50^{\circ})$
$\implies I^{\prime \prime}=(0.330W/m^2)(0.821)=272mW/m^2$
