Let $I_0$ be the intensity of incident light. As the light coming from sodium lamp is unpolarized, so the intensity of light emerging from the first polaroid is

$I_1=\frac{I_0}{2}$

If $\theta$ is the angle between two polaroids, then the intensity of light emerging from the second polaroid is

$I_2=I_1{\mathrm{c}\mathrm{o}\mathrm{s}}^2\theta =\frac{I_0}{2}{\mathrm{c}\mathrm{o}\mathrm{s}}^2\theta$

But $I_0=100\mathrm{\%}$ (given)

$\therefore I_2=\left(50\mathrm{\%}\right){\cos }^2\theta$

Since $\theta$ varies from $90°$ to $0°$, so the intensity of the outcoming light can be varied from $0\mathrm{\%}$ to $50\mathrm{\%}$.