After first polaroid: \(I_0\). After middle polaroid (at \(22.5°\)): \(I_1 = I_0\cos^2 22.5°\). The remaining angle to the second crossed polaroid is \(90°-22.5°=67.5°\). After second: \(I = I_1\cos^2 67.5° = I_0\cos^2 22.5°\cos^2 67.5°\). Using \(\cos22.5°\cos67.5° = \frac{1}{2}\sin45° = \frac{\sqrt{2}}{4}\): \(I = I_0\cdot\frac{1}{4}\cdot\sin^2 45° = I_0\cdot\frac{1}{4}\cdot\frac{1}{2} = \frac{I_0}{8}\).